This study has several clear-cut practical implications. First, consideration should be paid to the definitive ratios of elemental composition of important body compartments when formulating the concentrations and proportions of the elements in a hyperalimentation fluid. Some clinicians may use infusion mixtures, usually on a short-term basis, which lack one or other of the elements studied by the authors. The ex peri ment s iIIust r at e d iff er e nt patterns of weight gain that may occur during intravenous feeding and show how some may be more beneficial to the patient than others. It is helpful to have the consequences of such practice demonstrated. 0
1. S.J. Dudrick, J.M. Long, E. Steiger and J.E. Rhoads: Intravenous Hyperalimentation. Med. Clin. North Am. 54: 577-589, 1970 2. D. Rudman, W.J. Milliken, T.J. Richardson,T.J. Bixler II, W.J. Stackhouse and W.C. McGarrity: Elemental Balances During Intravenous Hyperalimentation. J. Clin. Invest. 55: 94-104, 1975 3. E.J. Reifenstein Jr., F. Albright and S.J. Wells: The Accumulation, Interpretation, and Presentation of Data Pertaining to Metabolic Balances, Notably those of Calcium, Phosphorus, and Nitrogen.J. Clin. Endocrinol. Metab. 5367395, 1945
IN WHAT FORM DOES VITAMIN B6 EXIST IN PLASMA? Following administration of pyridoxine to man or animals, pyridoxal phosphate appears in the plasma. The main source of this vitamer appears to be the liver. It circulates complexed to albumin; this makes it less susceptible to enzyme degradation. Key Words: vitamin Bg, pyridoxal, pyridoxal phosphate, erythrocyte, liver
The most physiologically active form of vitamin B g is pyridoxal phosphate. The form in which it predominantly exists in plasma is, however, controversial. While Sauberlich et al.1 categorically stated it to be pyridoxal phosphate,Anderson et al.273 considered it to be pyridoxal. The source from which plasma mainly derives the vitamin is also not clearly established. Lumeng and co-workers4 recently conducted studies which showed that plasma contains significant amounts of pyridoxal phosphate. The liver appears to be the primary source of this vitamer. Three healthy adult males were given oral supplements of 25 mg of pyridoxine daily for six to eight days. Their blood was sampled at one to two day intervals for 15 to 20 days. There was a significant increase in plasma pyridoxal phosphate concentration, which decreased promptly following withdrawal of the vitamin. The studies of Anderson et aI.* indicated that pyridoxal phosphate cannot enter erythrocytes and these workers suggested that a firm binding to a plasma protein might prevent this entry. The present study shows that pyridoxal 40 NUTRITION REVIEWSIVOL. 34, NO. 2IFEBRUARY 1976
phosphate in human plasma is bound preferentially to albumin and that this binding may prevent the entry into cells. It may also make the vitamer less susceptible to degradation by phosphatases. In the next experiment, either pyridoxine or pyridoxal was injected into anesthetized dogs and blood was sampled at 15 minute intervals. A prompt rise in plasma pyridoxal phosphate concentration was observed over the 90 minute period of the experiment. This response was not affected either by bilateral nephrectomy or by the resection of the spleen, stomach and intestines. When hepatectomy was performed, however, either singly or in combination with the resection of other splanchnic organs, there was virtually no increase in pyridoxal phosphate concentration. This suggests that the liver might be the main source of plasma pyridoxal phosphate. The investigations by Anderson et al. indicated that pyridoxal may be the main vitamer in plasma whose release may follow hydrolysis of pyridoxal phosphate in the erythrocyte. These studies were handicapped by the fact that they were performed in vitro. Hence they can only indicate the ability of the erythrocyte to contribute pyridoxal to plasma but do not neces-
sarily pinpoint it to be the main source. Nevertheless, these two important investigations may be considered complementary to each other. Anderson et al. demonstrated that pyridoxal phosphate cannot leave the erythrocytes and this observation has been corroborated by the present authors. Possibly the phosphorylated vitamin cannot leave other cells either, with the exception of the liver. Lumeng et al. suggested that it may be released from the liver complexed to albumin. It may then be inferred that plasma pyridoxal phosphate is principally derived from the liver whereas the erythrocytes may contribute to the plasma pyridoxal pool. Albumin-bound pyridoxal phosphate being virtually nontransportable into the cells may help in maintaining an equilibrium between the plasma and tissue concentrations of the vitamin. Clearly more studies are needed to understand the signifi-
cance of the presence in plasma of this coenzyme form of vitamin Bg.
1. H.E. Sauberlich, J.E. Canham, E.M. Baker, N. Raica, Jr. and Y.F. Herman: Biochemical Assessment of the Nutritional Status of Vitamin B.5 in the Human. Am. J. Clin. Nutrition 25: 629-642, 1972 2. B.B. Anderson, C.E. Fulford-Jones, J.A. Child, M.E.J. Beard and C.J.T. Bateman: Conversion of Vitamin B6 Compounds to Active Forms in the Red Blood Cell. J. Clin. Invest. 50: 19011909, 1971 3. Conversion of Vitamin B6 Compounds in Human Red Blood Cells. Nutrition Reviews 30: 119-121, 1972 4. L. Lumeng, R. E. Brashear and T. K. Li: Pyridoxal 5'-phosphate in Plasma: Source Protein Binding, and Cellular Transport. J. Lab. Clin. Med. 84: 334-343,1974
- KETO ANALOGUES OF ESSENTIAL AMINO ACIDS IN TREATMENT OF HUMAN DISEASES The utility of providing keto analogues of essential amino acids in several clinical conditions is being investigated. Key Words: essential amino acids, keto acid analogues, nitrogen balance, uremia, congential hyperammonemia, carbamyl phosphate synthetase
Walser, in a series of investigations over the past five years, synthesized production quantities of the alpha-keto analogues of valine, leucine, isoleucine, methionine and phenylalanine and employed them in a series of animal and human experiments. These materials, when given in quantities similar to the intake of the essential amino acid, attach labile nitrogen to become essential amino acids. Since most essential amino acids can be replaced by their alpha-keto analogues in the diet, further investigation of their role in disease seemed in order. Study of perfused rat liver172 showed that the liver rapidly uses up 49 to 155 pmole per hour per 200 g rat and that the corresponding amino acids appeared in the medium. Despite this increased conversion urea release was unal-
tered. Study of the source of the nitrogen indicated that nearly all came from glutamine. Hindquarter perfusion (mostly muscle) resulted in linear disappearance of keto acid at about the same maximal rates as found for the liver. During perfusion the corresponding amino acids increased in the tissue but all other essential amino acid concentrations fell. In muscle the source of the nitrogen was not clearly identified but it was not glutamine. In a subsequent study in man3 the nitrogen sparing effect of the keto acids of these same five essential amino acids was investigated during prolonged starvation in obese subjects. Eleven obese women were studied. Six of the women were studied after 33 days of fasting and a steady state of urea nitrogen excretion in the urine was established. Seven daily infusions of the five keto acids noted above and the other essential amino acids making up approximately the minimal daily requirement of all essential amino acids were given. Daily urea NUTRITION REVIEWSIVOL. 34 NO. 2IFEBRUARY 1976 41
1. S.J. Dudrick, J.M. Long, E. Steiger and J.E. Rhoads: Intravenous Hyperalimentation. Med. Clin. North Am. 54: 577-589, 1970 2. D. Rudman, W.J. Milliken, T.J. Richardson,T.J. Bixler II, W.J. Stackhouse and W.C. McGarrity: Elemental Balances During Intravenous Hyperalimentation. J. Clin. Invest. 55: 94-104, 1975 3. E.J. Reifenstein Jr., F. Albright and S.J. Wells: The Accumulation, Interpretation, and Presentation of Data Pertaining to Metabolic Balances, Notably those of Calcium, Phosphorus, and Nitrogen.J. Clin. Endocrinol. Metab. 5367395, 1945
IN WHAT FORM DOES VITAMIN B6 EXIST IN PLASMA? Following administration of pyridoxine to man or animals, pyridoxal phosphate appears in the plasma. The main source of this vitamer appears to be the liver. It circulates complexed to albumin; this makes it less susceptible to enzyme degradation. Key Words: vitamin Bg, pyridoxal, pyridoxal phosphate, erythrocyte, liver
The most physiologically active form of vitamin B g is pyridoxal phosphate. The form in which it predominantly exists in plasma is, however, controversial. While Sauberlich et al.1 categorically stated it to be pyridoxal phosphate,Anderson et al.273 considered it to be pyridoxal. The source from which plasma mainly derives the vitamin is also not clearly established. Lumeng and co-workers4 recently conducted studies which showed that plasma contains significant amounts of pyridoxal phosphate. The liver appears to be the primary source of this vitamer. Three healthy adult males were given oral supplements of 25 mg of pyridoxine daily for six to eight days. Their blood was sampled at one to two day intervals for 15 to 20 days. There was a significant increase in plasma pyridoxal phosphate concentration, which decreased promptly following withdrawal of the vitamin. The studies of Anderson et aI.* indicated that pyridoxal phosphate cannot enter erythrocytes and these workers suggested that a firm binding to a plasma protein might prevent this entry. The present study shows that pyridoxal 40 NUTRITION REVIEWSIVOL. 34, NO. 2IFEBRUARY 1976
phosphate in human plasma is bound preferentially to albumin and that this binding may prevent the entry into cells. It may also make the vitamer less susceptible to degradation by phosphatases. In the next experiment, either pyridoxine or pyridoxal was injected into anesthetized dogs and blood was sampled at 15 minute intervals. A prompt rise in plasma pyridoxal phosphate concentration was observed over the 90 minute period of the experiment. This response was not affected either by bilateral nephrectomy or by the resection of the spleen, stomach and intestines. When hepatectomy was performed, however, either singly or in combination with the resection of other splanchnic organs, there was virtually no increase in pyridoxal phosphate concentration. This suggests that the liver might be the main source of plasma pyridoxal phosphate. The investigations by Anderson et al. indicated that pyridoxal may be the main vitamer in plasma whose release may follow hydrolysis of pyridoxal phosphate in the erythrocyte. These studies were handicapped by the fact that they were performed in vitro. Hence they can only indicate the ability of the erythrocyte to contribute pyridoxal to plasma but do not neces-
sarily pinpoint it to be the main source. Nevertheless, these two important investigations may be considered complementary to each other. Anderson et al. demonstrated that pyridoxal phosphate cannot leave the erythrocytes and this observation has been corroborated by the present authors. Possibly the phosphorylated vitamin cannot leave other cells either, with the exception of the liver. Lumeng et al. suggested that it may be released from the liver complexed to albumin. It may then be inferred that plasma pyridoxal phosphate is principally derived from the liver whereas the erythrocytes may contribute to the plasma pyridoxal pool. Albumin-bound pyridoxal phosphate being virtually nontransportable into the cells may help in maintaining an equilibrium between the plasma and tissue concentrations of the vitamin. Clearly more studies are needed to understand the signifi-
cance of the presence in plasma of this coenzyme form of vitamin Bg.
1. H.E. Sauberlich, J.E. Canham, E.M. Baker, N. Raica, Jr. and Y.F. Herman: Biochemical Assessment of the Nutritional Status of Vitamin B.5 in the Human. Am. J. Clin. Nutrition 25: 629-642, 1972 2. B.B. Anderson, C.E. Fulford-Jones, J.A. Child, M.E.J. Beard and C.J.T. Bateman: Conversion of Vitamin B6 Compounds to Active Forms in the Red Blood Cell. J. Clin. Invest. 50: 19011909, 1971 3. Conversion of Vitamin B6 Compounds in Human Red Blood Cells. Nutrition Reviews 30: 119-121, 1972 4. L. Lumeng, R. E. Brashear and T. K. Li: Pyridoxal 5'-phosphate in Plasma: Source Protein Binding, and Cellular Transport. J. Lab. Clin. Med. 84: 334-343,1974
- KETO ANALOGUES OF ESSENTIAL AMINO ACIDS IN TREATMENT OF HUMAN DISEASES The utility of providing keto analogues of essential amino acids in several clinical conditions is being investigated. Key Words: essential amino acids, keto acid analogues, nitrogen balance, uremia, congential hyperammonemia, carbamyl phosphate synthetase
Walser, in a series of investigations over the past five years, synthesized production quantities of the alpha-keto analogues of valine, leucine, isoleucine, methionine and phenylalanine and employed them in a series of animal and human experiments. These materials, when given in quantities similar to the intake of the essential amino acid, attach labile nitrogen to become essential amino acids. Since most essential amino acids can be replaced by their alpha-keto analogues in the diet, further investigation of their role in disease seemed in order. Study of perfused rat liver172 showed that the liver rapidly uses up 49 to 155 pmole per hour per 200 g rat and that the corresponding amino acids appeared in the medium. Despite this increased conversion urea release was unal-
tered. Study of the source of the nitrogen indicated that nearly all came from glutamine. Hindquarter perfusion (mostly muscle) resulted in linear disappearance of keto acid at about the same maximal rates as found for the liver. During perfusion the corresponding amino acids increased in the tissue but all other essential amino acid concentrations fell. In muscle the source of the nitrogen was not clearly identified but it was not glutamine. In a subsequent study in man3 the nitrogen sparing effect of the keto acids of these same five essential amino acids was investigated during prolonged starvation in obese subjects. Eleven obese women were studied. Six of the women were studied after 33 days of fasting and a steady state of urea nitrogen excretion in the urine was established. Seven daily infusions of the five keto acids noted above and the other essential amino acids making up approximately the minimal daily requirement of all essential amino acids were given. Daily urea NUTRITION REVIEWSIVOL. 34 NO. 2IFEBRUARY 1976 41
