I-Medlcal Mdd
Hlprknr
~~~
Hypotheses
(1992) 39. su-l
0~orapuKLad1992
Ascorbate Status and Xerostomia C.O. ENWONWU Center for Nutrition Research and Department Nashville, TN 37208, USA
of Family and Preventive
Medicine,
Meharry Medical College,
Abstract-Xerostomia, the subjective feeling of dry mouth, affects millions of people particularly the elderly. It is invariably associated with hypofunction of the salivary glands. The amount, rate of secretion, and composition of saliva are regulated by both sympathetic and parasympathetic receptor systems whose stimulation transmits signals through intracellular messengers (cations, nucleotides, phospholipid derivatives) to structures and enzymes within the cell. Salivary glands express a variety of cell-surface receptors including adrenergic (aand j3), muscarinic-cholinergic, substance P, vasoactive intestinal peptide hormone, and ATP receptors. Ascorbate which is present in salivary acinar cells in relatively high concentrations, is closely involved in many cellular functions including the metabolism of pyrimidines, intracellular calcium, the catecholamines and other neurotransmitters which regulate salivary gland exocytosis. Ascorbate-dependent carboxyl-terminal peptide a-amidation enzyme similar to the pituitary peptidyl-glycine a-amidating monooxygase, is also present in salivary glands. It is therefore not fortuitous that the seemingly unrelated numerous factors like aging, drug ingestion, pregnancy, smoking, ionizing radiation, stress, and various pathological states such as cancer, autoimmune disorders, diabetes mellitus, and hypertension often implicated in the causation of xerostomia, all promote increased tissue requirement for and/or depletion of ascorbate.
Introduction
and oral health, particularly the susceptibility to dental caries and opportunistic infections (l-4). Dry mouth, encountered most frequently in the elderly population, is attributed by many investigators to such factors as ingestion of drugs, ionizing radiation, smoking, depression, physiological stress and several pathological states including autoimmuue disorders like the Sjogren’s syndrome, diabetes mellitus, hypertension, hyperlipoproteinemia, and human immunodeficiency virus (HIV) infection (2,3,4). Interestingly, all these conditions including aging, promote increased tissue requirement for ascorbate (5-9).
Xerostomia, the subjective feeling of oral dryness, is not a trivial complaint, and may in fact signal the presence of a serious underlying systemic health problem (1, 2). It is associated with prominent hyposalivation. Saliva contains water, electrolytes, proteins and glycoproteins, and is produced by the major and minor salivary glands. Xerostomia exerts serious negative effects on the victim’s quality of life, affecting dietary habits. nutritional status, speech, taste, tolerance to dental prostheses, psychological well-being, Date received 16 February 1992 Date accepted 1 April 1992
53
54 thus suggesting cellular ascorbate depletion as a unifying mechanism in the causation of impaired salivary gland exocytosis and consequently, oral dryness, by an extensive list of seemingly unrelated conditions. S&vary gland exocyiosis
Neurochemical and hormonal factors as well as dietary habits and nutritional status impact on exocytosis in the salivary glands (10-13). The amount, rate of secretion. and comnosition of saliva are regulated by both sympathetic &d parasympathetic systems (10). Salivary glands express a variety of cell-surface receptors including adrenergic (a and p), muscariniccholinergic, substance P, vasoactive intestinal peptide hormone WIP), and ATP receptors. The cell membrane receptors receive stimuli from neurotransmitters and transmit signals through appropriate intracellular messengers (cations, nucleotides, phospholipid derivatives) to structures and enzymes within the cells. The signal transduction mechanism involved in salivary gland regulated exocytosis is the subject of a recent detailed review (14). Norepinephrine (NE) release at nerve endings stimualtes both a- and P-adrenergic receptors with increase in CAMP (lo), while stimulation of al -adrenergic, muscarinic-cholinergic, and substance P-peptidergic membrane receptors elicits phosphatidyl-inositol 4, 5 bisphosphate (PIP)2 hydrolysis, IP3 generation and subsequent elevation of cytosolic Caz+ level (10, 14, 15). Physiologically, multiple receptor activations and interactions do occur (10, 15). Disease states, including malnutrition, which modify receptor number and characteristics, as well as the signal transductive events, will affect salivary gland exocrine function.
MEDKALHvKXHEsEs the controls, ascorbic acid deficient guinea pigs also showed significantly (p 4.01) reduced submandibular gland content of the vitamin, markedly increased levels of the free aromatic amino acids ‘Qr and Phe in the gland and serum, severe reduction in the activity (nmol mint g-1 tissue) of dopamine-P-monooxygenase cEC1.14.17.1) in the submandibular gland and a three-fold increase-in plasma corticosteroidlevel(l3). Tissue distribution and biological functions of
ascorbute
Ascorbic acid is an ubiquitous component of animal tissues where it occurs mainly in two forms, the dihydro-(reduced) and the dehydro-(oxidized) forms. Present in the tissues are enzymes glutathione: dehydro-L-ascorbate oxidoreductase (EC1 X5.1) and the NADPH-linked reductase which regenerate dihydroascorbate from the oxidized form (22). The highest concentrations of ascorbate are found in the adrenal and pituitary glands, key organs involved in maintenance of homeostasis (23). Hypertrophy and hyperplasia of the adrenal gland are consistent findings in ascorbic acid deficiency (24), and associated with these is a prominent increase in circulating free cortisol (6, 13). In view of the well documented excellent correlation between plasma and saliva levels of free cortisol(25), ascorbate deficiency should also promote increased level of cords01 in the salivary gland and saliva although the latter,has not been studied. High concentrations of ascorbate have also been reported in other glandular tissues like the salivary glands (26, 27). Indeed, following intravenous administration of (1-W jascorbate to guinea pigs, the fastest uptake rates and highest retention capacity of the vitamin are exhibited by the pituitary, adrenal and submandibular salivary glands (26, 27, 28). Studies Salivary gland in ascorbate deficiency (29) demonstrate that similar to the adrenal chtcPainful swelling of the salivary glands has been maffin granules, acinar secretory granules of the rat described in severe ascorbic acid deficiency (16). parotid gland contain dihydroascorbate at concentraAmong the prominent features of experimentally in- tions much higher than in plasma, and additionally, duced scurvy in the human are manifestations of the there is a distinct extragranular pool comprising as sicca syndrome as exemplified by xerostomia, en- much as 80% of the total cellular ascorbate. Ascorbic acid defies simple definition in terms of largement of the salivary glands, keratoconjunctivitis its multiple biologic functions which have been exand arthritis (17, 18). Studies in appropriate experimental animals have tensively reviewed (8,9,23, 30-34). Ascorbic acid is demonstrated prominent edema of the tissues around important for maintenance of integrity of cell memthe major salivary glands (19), marked atrophy of the branes and optimal immune function, as well as for salivary acinar cells with widening of the interlobu- the metabolism of several nutrients and nonnutrients, lar septa (20, 21), and an equally significant reduc- including drugs. Many roles of this important antition (-55% compared with animals pair-fed adequate oxidant nutrient derive from its ability to donate elecascorbate diet) in flow rate of stimulated saliva in trons while itself undergoing reversible oxidation (23, ascorbic acid deficient animals (13). Compared with 30, 33). Formation of the neurotransmitters NE and
ASCORBATE STATUS AND XEROSTOMIA
serotonin is influenced by ascorbate (32, 33). Ascorbic acid is required for optimal activity of the pituitary peptidyl-glycine a-amidating monooxygenase that functions in post-translational processing of secretory polypeptides within neural and endocrine secretion granules (31, 32, 34). The peptides that are am&ted by this enzyme include substance P and VIP (34), which are among the important well-studied neurotransmitters in salivary gland exocytosis (10). Perhaps of more relevance, although not yet fully explored, is the observation that acinar cell secretory granules from parotid gland contain a carboxylterminal peptide a-amidation enzyme with close resemblance to the pituitary enzyme in its ascorbate and copper requirements, pH dependence, and kinetic properties, as well as its predominant location in the secretory granules (35). Ascorbate promotes an increase in acetylcholine receptor number and distribution in certain cells and this is believed to occur through transcriptional regulation (36, 37). Similar to findings of acetylcholine and noradrenaline release from ascorbic acid-treated, Caz+ -stimualted rat brain synaptic vesicles, studies also demonstrate that cholinergic nerve terminals contain ascorbate which induces Ca2+ -dependent release of acetylcholine and ATP from isolated synaptic vesicles of Torpedo ocellata (38), an observation suggestive of a possible role for this vitamin in maintaining the balance between vesicular and cytoplasmic pools of acetylcholine within the nerve terminal. Equally relevant to the subject of this paper are reported observations of involvement of ascorbic acid in the metabolism of cyclic nucleotides (33, 39). This vitamin is also believed to function as an endogenous cytosolic inhibitor of ATP-supported mitochondrial Ca2+ -transport (40). Tenenhouse and Afari (41) suggest that mitochondrial Ca2+ uptake prevents an increase in cytoplasmic Ca2+ and thus antagonizes the effects of secretagogues on exocrine tissues. Conclusion The current officially recommended daily allowance for ascorbic acid in the US is 60 mg, with a suggested higher intake (100 mg/day) for smokers and an increment of 10 mg/day for pregnant women (42). Such values have however been challenged as very low (43). What is however clear is that many factors promote an increased requirement for this vitamin. Among these factors are various physiological stresses, smoking, exposure to ionizing radiation, drug ingestion, as well as a whole series of pathological states like diabetes mellitus, cancer, hyper-
55 tension, hypercholesterolemia, and autoimmune disorders (6-9, 3 1,44). Overproduction of reactive oxygen radicals is a major problem in many autoimmune diseases (45) and also during irradiation (6, 46). and these conditions consume high amounts of ascorbic acid (6, 30, 47). Tissue ascorbate level generally declines with aging, a phenomenon noted in the free-living elderly in several countries (5,6,30). Suter and Russell (5) have reported that up to 25% of the free-living elderly in the US are vitamin C deficient, and depending on race and income level, 23%-42% of the elderly surveyed in HANES I had ascorbic acid intakes less than 30 mg/day. The features of ascorbate depletion include hypofunction of the pancreas and thyroid (6) as well as very prominent increase in circulating levels of glucocorticoids (6, 13). These changes are reported to impair salivary gland exocytosis (11). Evaluation of the importance of adequate ascorbate status on salivary gland function should also take due cognizance of the vitamin’s interactions with other essential nutrients. Vitamin C has protective effects against deficiencies of vitamins A, D, and E, for example. Intestinal absorption of these fat-soluble vitamins is closely linked to the pathways of dietary lipid absorption in which the bile salts play an essential role. Studies by Ginter (8) in ascorbic acid deficient guinea pigs indicate significant reduction in cholesterol transformation into bile acids, resulting from impaired activity of hepatic microsomal cholesterol 7 a-hydroxylase (ECl. 14.13.17), the rate-limiting step in bile acid synthesis. Defective utilization of ingested viatmins A and D, regardless of its cause, is known to produce significant reduction in saliva flow rate (11, 41,48,49). Xerostomia, the subjective feeling of oral dryness, resulting from a severe reduction in saliva flow rate, is a common problem particularly among the aged (50). The most frequently reported causes of dry mouth include autoimmune disorders such as Sjogren’s syndrome, drugs that inhibit salivary gland exocytosis, irradiation to the salivary glands and several physiological as well as pathological conditions (1, 2, 50). All these observed causes of xerostomia are major ‘consumers’ of tissue ascorbate stores and they are indeed among the factors frequently reported to increase tissue requirements for this nutrient (6-9, 30, 44, 47). Ascorbic acid status has extensive interactions with the metabolism of several hormonesheurotransmitters as well as with the intracellular messengers invovled in signal transductive events in salivary gland exocytosis. These observa-
56 tions lend support to the hypothesis that cellular depletion of ascorbate serves as the common link beWeen the long list of seemingly unrelated conditions variously implicated in the genesis of xerostomia. Acknowledgements The preliminary work which led to the formulation of the hypothesis in this report was supported by Kraft General Foods Foundation, Glenview, Illinois. Preparation of this publication was supported in pan by NIH Grant ROl DEG9653-01 from NIDR. The author also wishes to acknowledge the excellent secretarial assistance of Mrs Iori Shamblin.
References 1. Fox PC, Weiffenbach J M. van der Ven P F. Baum B I, Sonies B C. Xerostomia: evaluation of a symptom with increasing significance. JADA 110.519-525, 1985. 2. Sreebnv L M. Valdini A. Yu A. Xerostomia. Part 11: Relationship to*ncnoral symptoms, drugs and diseases. Oral Surg Oral Med Oral Pathoi 68: 419-427,1989. 3 Fox P C. Saliva and salivarv gland alterations in HIV infection. JADA 122: 46-l8, 1991: 4. McCarthy G M, Mackie I D, Koval J, Sandhu H S, Daley T D. Factors associated with increased frequency of HIV-related oral candidiasis. J Oral Path01 Med 20: 332336, 1991. 5. Suter P M, Russell R M. Vitamin requirements of the elderly. Am J Clin Nutr 45: 501-512, 1987. 6. Clemetson C A B. Vitamin C. Vols 1, 2. 3. CRC Press Inc. Boca Raton. Florida, 1989. 7. Choi E S K. Jacques P F, Dallal G E. Jacob R A. Correlation of blood pressure with plasma ascorbic acid. Nutr Res 11: 1377-1382, 1991. 8. Ginter E. Cholesterol: vitamin C controls its transformation to bile acids. Science 179: 702-704, 1973. 9. Gaby S K. Singh V N. Vitamin C. ~103 in Vitamin Intake and Health. A Scientific Review. (S K Gaby, A Bendich, V N Singh, L J Machlin, eds) Marcel Dekker, New York, 1991. control of secretion. J Dent Res 10. Baum B J. Neurotransmitter 66: 628-632, 1987. 11. Johnson D A. Regulation of salivary glands and their secretions by masticatory, nutritional, and hormonal factors. ~135 in Tbe Salivary System. (L M Sreebny, ed) CRC Press, Boca Raton. Florida, 1987. 12. Enwonwu C 0. Biochemical and morphologic changes in rat submandibular gland in experimental protein-calorie malnutrition. Exp Molec Path01 16: 244-269, 1972. 13. Enwonwu C 0. Hypofunction of salivary glands in ascorbic acid deficiency. Med Sci Res 18: 353-354. 1990. 14. QuisselJ D 0, Watson E, Dowd F J. Signal transduction mechanisms involved in salivary gland regulated exocytosis. Crit Rev Oral Bio13: 83-107, 1992. calcium entry. 15. Putney J W. A model for receptor-regulated Cell Calcium 7: l-12, 1986. 16. O’Shea H V. Scurvy. Practitioner 101, 217 and 283. 1918. 17. Hood J. Bums M A. Hodaes R E. Siiiaren’s syndrome in scurvy. N Engl J Med 282:?12&1124: 1970. _ 18. Hodges R E. What’s new about scurvy? Am J Clin Nutr, 24: 383-384. 1971. malnutrition in 19. Enwonwu C 0. Experimental protein-calorie the guinea pig and evaluation of the role of ascorbic acid status. Lab Investigation 29: 17-26, 1973.
MEDICAL HYPo.nDms
20. Toverud G. The influence of diet on teeth and bones. J Biol Chem 58: 583-600, 1923. 21. Hijjer J A. Studies in scurvy. Acta Paediat (Suppl2): 8-278, 1924. 22. Choi J L, Rose R C. Regeneration of ascorbic acid by rat colon. Prac Sot EXD Biol Med 190: 368-374.1989. 23. Levine M, Morita K. Ascorbic acid in endocrine systems. Vitamins and Homrones. 42: l-64, 1985. 24. Chalopin H. Moutcn M, Ratsimamanga A R. Some interrelations between ascorbic acid and adreno-cortical function. Wld Rev Nutr Diet 6: 165-196. 1966. 25. Vining R F, McGinley R A. The measurement of hormones in saliva: possibilities and pitfalls. J Steroid Biochem. 27: 81-94, 1987. 26. Homig D. Distribution of ascorbic acid, mr&abolites. and analogues in man and animals. Ann NY Acad Sci 258: 103-117, 1975. 27. Homig D, Weber F. Wiss 0. Autoradiographic distribution of (l-14C) Ascorbic acid and (1-14C) Dehydroascorbic acid in male guinea pigs after intravenous injection. Intemat J Vit Nutr Res 42: 223-241, 1972. 28. Homig D, Hartman D. Kinetic behavior of ascorbic acid in guinea pigs. ~293 in Ascorbic Acid: Chemistry, Metabolism and Uses. (P A Seib, B M Tolbert, eds), Advances in Chemistry Series 200, Amer Chern Sot. Washington D C, 1982. 29. von Zastrow M. Trittcn T R, Castle J D. Identification of Lascorbic acid in secretion granules of the rat parotid gland. J Biol Chem 259: 11746-11750. 1984. 30. Moser U. Bencich A. Vitamin C. ~195 in Handbook of Vitamins, Second Edition (L J Machlin, ed), Marcel Decker, Inc, New York, 1991. 31. Padh H. Cellular functions of ascorbic acid. Biochem Cell Biol 68: 11661173. 1990. 32. Levine M. New concepts in the biology and biochemistry of ascorbic acid. N Engl J Med 314: 892902, 1986. 33. Lewin S. Vitamin C: Its Molecular Biology and Medical Potential. Academic Press, London, 1976. 34. Englard S, Seifter S. The biochemical functions of ascorbic acid. Annu Rev Nutr 6: 365406. 1986. 35. von Zastrow M, Tritton T R, Castle J D. Exocrine secretion granules contain peptide amidation activity. Proc Natl Acad Sci USA 83: 3297-3301, 1986. 36. Knaack D, Shen I, Salpeter M M. Podleski T R. Selective effects of ascorbic acid on acetylcholine receptor number and distribution. Proc Natl Acad Sci USA, 82: 575-579, 1985. 37. Anonymous. Ascorbic acid increases the density of the acetylcholine receptor on muscle cells. Nutr Rev 47: 378-379,1989. 38. Pinchasi I, Michaelscm D M, Sokolovsky M. Cholinetgic nerve terminals contain ascorbic acid which induces C&+ dependent release of acetylcholine and ATP from isolated Torpedo synaptic vesicles. FEBS Letts 108: 189-192, 1979. 39. Buck M G, Zadunaisky. Stimulation of ion transport by ascorbic acid through inhibition of 3’:5’-cyclic-AMP phosphodiesterase in the cornea1 &helium and other tissues. Biochim Biophys Acta 389: 251-260. 1975. 40. Levine H, Bronson D D, Khouri R, Sahyoun N E. Ascorbic acid is an endogencus cytosolic inhibitor of ATP-supported rat liver mitochondrial calcium transport J Biol Chem 258: 14954-14959. 1983. 41. Tenenhouse A. Afari G. Protein secretion from parotid glands of vitamin D deficient and glucocorticoid-treated rats. Biochim Bicphys Acta 538: 631-634, 1978. Dietary Allow42. National Research Council. Recommended ances.10th edition. National Academic Press, Washington, DC., 1989.
57
ASCORBATE STATUS AND XEROSTOMIA
43. 44. 45. 46.
47.
Pauling L. Evolution and the need for ascorbic acid. hoc Natl Acad Sci, USA 67: 1643-1648, 1970. Murata A. Smoking and vitamin C. Wld Rev Nutr Diet 64: 31-57.1991. Pryor W, Mamett L. Free radicals: research on biochemical bad boys canes of age. J NIH Res 1: 101-106, 1989. Nishi M, Takashima H. Oka T, Ohishi N. Yagi K. EfTect of Xray irradiation on lipid peroxide levels in the rat suhnandibular gland. J Dent Res 65: 1028-1029, 1986. Niki E. Vitamin C as an antioxidant. Wld Rev Nutr Diet 64: l-30.1991.
48.
Johansson
I. Eriuon
M. ‘lhe ei%ct of vitamin A two salivary glycotxoteins in the adult rat. Internal J Vit Nutr Rea 59: 234-235. i989. Arwno M A, Lamb A J. OLwn J A. impaired salivary gland secretory function following the induaion of rapid. synchmnous vitamin A deficiency in rats. J NW 111: 49&504, 1981. Sreebny L M, Valdini A. Xerortomia. A neglected symptom. Arch Intern Med 147: 1333-1337. 1987. dehiency on the
49.
50.
T, Lumikari
secretimof salivaand
Hlprknr
~~~
Hypotheses
(1992) 39. su-l
0~orapuKLad1992
Ascorbate Status and Xerostomia C.O. ENWONWU Center for Nutrition Research and Department Nashville, TN 37208, USA
of Family and Preventive
Medicine,
Meharry Medical College,
Abstract-Xerostomia, the subjective feeling of dry mouth, affects millions of people particularly the elderly. It is invariably associated with hypofunction of the salivary glands. The amount, rate of secretion, and composition of saliva are regulated by both sympathetic and parasympathetic receptor systems whose stimulation transmits signals through intracellular messengers (cations, nucleotides, phospholipid derivatives) to structures and enzymes within the cell. Salivary glands express a variety of cell-surface receptors including adrenergic (aand j3), muscarinic-cholinergic, substance P, vasoactive intestinal peptide hormone, and ATP receptors. Ascorbate which is present in salivary acinar cells in relatively high concentrations, is closely involved in many cellular functions including the metabolism of pyrimidines, intracellular calcium, the catecholamines and other neurotransmitters which regulate salivary gland exocytosis. Ascorbate-dependent carboxyl-terminal peptide a-amidation enzyme similar to the pituitary peptidyl-glycine a-amidating monooxygase, is also present in salivary glands. It is therefore not fortuitous that the seemingly unrelated numerous factors like aging, drug ingestion, pregnancy, smoking, ionizing radiation, stress, and various pathological states such as cancer, autoimmune disorders, diabetes mellitus, and hypertension often implicated in the causation of xerostomia, all promote increased tissue requirement for and/or depletion of ascorbate.
Introduction
and oral health, particularly the susceptibility to dental caries and opportunistic infections (l-4). Dry mouth, encountered most frequently in the elderly population, is attributed by many investigators to such factors as ingestion of drugs, ionizing radiation, smoking, depression, physiological stress and several pathological states including autoimmuue disorders like the Sjogren’s syndrome, diabetes mellitus, hypertension, hyperlipoproteinemia, and human immunodeficiency virus (HIV) infection (2,3,4). Interestingly, all these conditions including aging, promote increased tissue requirement for ascorbate (5-9).
Xerostomia, the subjective feeling of oral dryness, is not a trivial complaint, and may in fact signal the presence of a serious underlying systemic health problem (1, 2). It is associated with prominent hyposalivation. Saliva contains water, electrolytes, proteins and glycoproteins, and is produced by the major and minor salivary glands. Xerostomia exerts serious negative effects on the victim’s quality of life, affecting dietary habits. nutritional status, speech, taste, tolerance to dental prostheses, psychological well-being, Date received 16 February 1992 Date accepted 1 April 1992
53
54 thus suggesting cellular ascorbate depletion as a unifying mechanism in the causation of impaired salivary gland exocytosis and consequently, oral dryness, by an extensive list of seemingly unrelated conditions. S&vary gland exocyiosis
Neurochemical and hormonal factors as well as dietary habits and nutritional status impact on exocytosis in the salivary glands (10-13). The amount, rate of secretion. and comnosition of saliva are regulated by both sympathetic &d parasympathetic systems (10). Salivary glands express a variety of cell-surface receptors including adrenergic (a and p), muscariniccholinergic, substance P, vasoactive intestinal peptide hormone WIP), and ATP receptors. The cell membrane receptors receive stimuli from neurotransmitters and transmit signals through appropriate intracellular messengers (cations, nucleotides, phospholipid derivatives) to structures and enzymes within the cells. The signal transduction mechanism involved in salivary gland regulated exocytosis is the subject of a recent detailed review (14). Norepinephrine (NE) release at nerve endings stimualtes both a- and P-adrenergic receptors with increase in CAMP (lo), while stimulation of al -adrenergic, muscarinic-cholinergic, and substance P-peptidergic membrane receptors elicits phosphatidyl-inositol 4, 5 bisphosphate (PIP)2 hydrolysis, IP3 generation and subsequent elevation of cytosolic Caz+ level (10, 14, 15). Physiologically, multiple receptor activations and interactions do occur (10, 15). Disease states, including malnutrition, which modify receptor number and characteristics, as well as the signal transductive events, will affect salivary gland exocrine function.
MEDKALHvKXHEsEs the controls, ascorbic acid deficient guinea pigs also showed significantly (p 4.01) reduced submandibular gland content of the vitamin, markedly increased levels of the free aromatic amino acids ‘Qr and Phe in the gland and serum, severe reduction in the activity (nmol mint g-1 tissue) of dopamine-P-monooxygenase cEC1.14.17.1) in the submandibular gland and a three-fold increase-in plasma corticosteroidlevel(l3). Tissue distribution and biological functions of
ascorbute
Ascorbic acid is an ubiquitous component of animal tissues where it occurs mainly in two forms, the dihydro-(reduced) and the dehydro-(oxidized) forms. Present in the tissues are enzymes glutathione: dehydro-L-ascorbate oxidoreductase (EC1 X5.1) and the NADPH-linked reductase which regenerate dihydroascorbate from the oxidized form (22). The highest concentrations of ascorbate are found in the adrenal and pituitary glands, key organs involved in maintenance of homeostasis (23). Hypertrophy and hyperplasia of the adrenal gland are consistent findings in ascorbic acid deficiency (24), and associated with these is a prominent increase in circulating free cortisol (6, 13). In view of the well documented excellent correlation between plasma and saliva levels of free cortisol(25), ascorbate deficiency should also promote increased level of cords01 in the salivary gland and saliva although the latter,has not been studied. High concentrations of ascorbate have also been reported in other glandular tissues like the salivary glands (26, 27). Indeed, following intravenous administration of (1-W jascorbate to guinea pigs, the fastest uptake rates and highest retention capacity of the vitamin are exhibited by the pituitary, adrenal and submandibular salivary glands (26, 27, 28). Studies Salivary gland in ascorbate deficiency (29) demonstrate that similar to the adrenal chtcPainful swelling of the salivary glands has been maffin granules, acinar secretory granules of the rat described in severe ascorbic acid deficiency (16). parotid gland contain dihydroascorbate at concentraAmong the prominent features of experimentally in- tions much higher than in plasma, and additionally, duced scurvy in the human are manifestations of the there is a distinct extragranular pool comprising as sicca syndrome as exemplified by xerostomia, en- much as 80% of the total cellular ascorbate. Ascorbic acid defies simple definition in terms of largement of the salivary glands, keratoconjunctivitis its multiple biologic functions which have been exand arthritis (17, 18). Studies in appropriate experimental animals have tensively reviewed (8,9,23, 30-34). Ascorbic acid is demonstrated prominent edema of the tissues around important for maintenance of integrity of cell memthe major salivary glands (19), marked atrophy of the branes and optimal immune function, as well as for salivary acinar cells with widening of the interlobu- the metabolism of several nutrients and nonnutrients, lar septa (20, 21), and an equally significant reduc- including drugs. Many roles of this important antition (-55% compared with animals pair-fed adequate oxidant nutrient derive from its ability to donate elecascorbate diet) in flow rate of stimulated saliva in trons while itself undergoing reversible oxidation (23, ascorbic acid deficient animals (13). Compared with 30, 33). Formation of the neurotransmitters NE and
ASCORBATE STATUS AND XEROSTOMIA
serotonin is influenced by ascorbate (32, 33). Ascorbic acid is required for optimal activity of the pituitary peptidyl-glycine a-amidating monooxygenase that functions in post-translational processing of secretory polypeptides within neural and endocrine secretion granules (31, 32, 34). The peptides that are am&ted by this enzyme include substance P and VIP (34), which are among the important well-studied neurotransmitters in salivary gland exocytosis (10). Perhaps of more relevance, although not yet fully explored, is the observation that acinar cell secretory granules from parotid gland contain a carboxylterminal peptide a-amidation enzyme with close resemblance to the pituitary enzyme in its ascorbate and copper requirements, pH dependence, and kinetic properties, as well as its predominant location in the secretory granules (35). Ascorbate promotes an increase in acetylcholine receptor number and distribution in certain cells and this is believed to occur through transcriptional regulation (36, 37). Similar to findings of acetylcholine and noradrenaline release from ascorbic acid-treated, Caz+ -stimualted rat brain synaptic vesicles, studies also demonstrate that cholinergic nerve terminals contain ascorbate which induces Ca2+ -dependent release of acetylcholine and ATP from isolated synaptic vesicles of Torpedo ocellata (38), an observation suggestive of a possible role for this vitamin in maintaining the balance between vesicular and cytoplasmic pools of acetylcholine within the nerve terminal. Equally relevant to the subject of this paper are reported observations of involvement of ascorbic acid in the metabolism of cyclic nucleotides (33, 39). This vitamin is also believed to function as an endogenous cytosolic inhibitor of ATP-supported mitochondrial Ca2+ -transport (40). Tenenhouse and Afari (41) suggest that mitochondrial Ca2+ uptake prevents an increase in cytoplasmic Ca2+ and thus antagonizes the effects of secretagogues on exocrine tissues. Conclusion The current officially recommended daily allowance for ascorbic acid in the US is 60 mg, with a suggested higher intake (100 mg/day) for smokers and an increment of 10 mg/day for pregnant women (42). Such values have however been challenged as very low (43). What is however clear is that many factors promote an increased requirement for this vitamin. Among these factors are various physiological stresses, smoking, exposure to ionizing radiation, drug ingestion, as well as a whole series of pathological states like diabetes mellitus, cancer, hyper-
55 tension, hypercholesterolemia, and autoimmune disorders (6-9, 3 1,44). Overproduction of reactive oxygen radicals is a major problem in many autoimmune diseases (45) and also during irradiation (6, 46). and these conditions consume high amounts of ascorbic acid (6, 30, 47). Tissue ascorbate level generally declines with aging, a phenomenon noted in the free-living elderly in several countries (5,6,30). Suter and Russell (5) have reported that up to 25% of the free-living elderly in the US are vitamin C deficient, and depending on race and income level, 23%-42% of the elderly surveyed in HANES I had ascorbic acid intakes less than 30 mg/day. The features of ascorbate depletion include hypofunction of the pancreas and thyroid (6) as well as very prominent increase in circulating levels of glucocorticoids (6, 13). These changes are reported to impair salivary gland exocytosis (11). Evaluation of the importance of adequate ascorbate status on salivary gland function should also take due cognizance of the vitamin’s interactions with other essential nutrients. Vitamin C has protective effects against deficiencies of vitamins A, D, and E, for example. Intestinal absorption of these fat-soluble vitamins is closely linked to the pathways of dietary lipid absorption in which the bile salts play an essential role. Studies by Ginter (8) in ascorbic acid deficient guinea pigs indicate significant reduction in cholesterol transformation into bile acids, resulting from impaired activity of hepatic microsomal cholesterol 7 a-hydroxylase (ECl. 14.13.17), the rate-limiting step in bile acid synthesis. Defective utilization of ingested viatmins A and D, regardless of its cause, is known to produce significant reduction in saliva flow rate (11, 41,48,49). Xerostomia, the subjective feeling of oral dryness, resulting from a severe reduction in saliva flow rate, is a common problem particularly among the aged (50). The most frequently reported causes of dry mouth include autoimmune disorders such as Sjogren’s syndrome, drugs that inhibit salivary gland exocytosis, irradiation to the salivary glands and several physiological as well as pathological conditions (1, 2, 50). All these observed causes of xerostomia are major ‘consumers’ of tissue ascorbate stores and they are indeed among the factors frequently reported to increase tissue requirements for this nutrient (6-9, 30, 44, 47). Ascorbic acid status has extensive interactions with the metabolism of several hormonesheurotransmitters as well as with the intracellular messengers invovled in signal transductive events in salivary gland exocytosis. These observa-
56 tions lend support to the hypothesis that cellular depletion of ascorbate serves as the common link beWeen the long list of seemingly unrelated conditions variously implicated in the genesis of xerostomia. Acknowledgements The preliminary work which led to the formulation of the hypothesis in this report was supported by Kraft General Foods Foundation, Glenview, Illinois. Preparation of this publication was supported in pan by NIH Grant ROl DEG9653-01 from NIDR. The author also wishes to acknowledge the excellent secretarial assistance of Mrs Iori Shamblin.
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