Clinical Endocrinology (1977) 7 . Suppl., IS-] 7 s .
VITAMIN D METABOLISM H E C T O R F. D E L U C A Department of Biochemishy, College o f Agricultural Life Sciences, University of Wisconsin-Madison,Madison, Wisconsin, U.S.A.
SUMMARY
During the past decade, an explosion of information has become avdable on the metabolism and function of vitamin D w h c h is of great importance to clinicians in the treatment of metabolic bone disease. We have learned that vltamin D is the precursor of at least one hormone and that t h s hormone carries out functions in calcium and phosphorus metabolism brmging about mineralization of bone on one hand, and the prevention of hypocalcaemic tetany on the other. It may also function in the prevention of such degenerative bone diseases as osteoporosis. An important analogue of this hormone, la-hydroxyvitamin D3 has been prepared and is used successfully in the treatment of a variety of clinical conditions. This presentation wdl summarize these findings and their possible implications in these metabolic bone diseases. I t is well known that vitamin D brings about the mineralization of organic matrix elaborated by both chondrocytes and osteoblasts (Sebrell & Harris. 1954). In so doing, it prevents the disease, rickets, in the young and osteomalacia in the adult. It also functions in the prevention of hypocalcaemic tetany, a very critical function which it shares with the parathyroid hormone (DeLuca, 1974; Rasmussen et al:. 1963; Harrison & Harrison. 1964). Because vitamin D is essential for intestinal calcium absorption and because its presence is necessary for the intestine t o adapt its efficiency of calcium absorption to the calcium needs of the organism, vitamin D plays a role in the prevention of bone loss or osteoporosis (DeLuca, 1976, 1977). Finally, it is known that vitamin D improves muscle strength, although the mechanism of this function is not understood even at the physiological level and remains to be studied. Figure 1 summarizes the sites at which vitamin D is known t o function in the prevention of the above described pathological conditions. The failure of bone mineralization in vitamin D deficiency is for the most part the result of an insufficient supply of calcium and phosphorus to the calcification centres (DeLuca, 1967). Although there is some sentiment that vitamin D must function directly in the mineralization process, so far a clear experimental demonstration of this possibility has not appeared. We, therefore, leave open the Correspondence: Dr H . F. DeLuca. Department of Biochemistry, 420 Henry Mall, University of Wisconsin-Madison, 'Cladison, Wisconsin 53706, U.S.A. IS
2s
H. F. DeLuca
distinct possibility that some form of vitamin D may function in that process. To prevent hypocalcaemic tetany and to provide sufficient amounts of calcium and phosphorus for minerahzation, it is well known that vitamin D functions to elevate both plasma calcium and plasma phosphate concentrations. To provide the necessary calcium and phosphorus, vitamin D stimulates at least three transport systems; the best known is the system which transports calcium across intestinal wall against an electro-chemical potential gradient (Nicolaysen & Eeg-Larsen, 1953; Schachter & Rosen, 1959; Wasserman, 1963; OmdaN & DeLuca, 1973). This process is stimulated primarily by vitamin D and indirectly by the parathyroid hormone as will be discussed later. Phosphate is the normal accompanying anion but in addition to this process, vitamin D initiates a phosphate transport reaction which is easily demonstrable in the distal portions of the small intestine (Harrison & Harrison, 1961 ; Kowarski & Schachter, 1969; Chen er af., 1974). Phosphate is transported against an electrochemical potential gradient (Walling, 1977), in a sodium dependent (Taylor, 1974) and calcium independent process (Harrison & Harrison, 1961 ; Kowarslu & Schachter, 1969; Chen et af., 1974). Thus there are two mechanisms in the small intestine whch contribute plasma calcium and phosphate to the extracellular fluid compartment. An extremely important source of calcium and phosphorus is previously mineralized bone. It must be recognized that the mineral phase of bone is separated from the extracellular fluid compartment by a membrane of cells, usually believed to be the osteoblastic and osteoclastic membrane barriers (Holtrop & Weinger, 1972; Talmage er al., 1975). I t is believed that vitamin D together with the parathyroid hormone stimulates the transfer of calcium from the bone fluid compartment to the extracellular fluid compartment (Rasmussen er al., 1963; Garabedian et al., 1974). At physiological concentrations both agents are required b u t the amount of each required can be diminished by a large excess of the other (Rasmussen ef al., 1963). This process is probably extremely important in the minute-to-minute control of plasma calcium concentration.
b d PI
Fig. 1. Physiological sites of action of 1,2540H),D, and the parathyroid hormone in their regulation of calcium and phosphorus metablism.
Vitamin D metabolism
3s
Another site of vitamin D action is the kidney. It has recently been demonstrated that vitamin D does improve renal reabsorption of calcium (Steele er al., 1975) and it is well known that the parathyroid hormone also improves renal reabsorption of calcium (Kleeman er al., 1961). It is not known how interdependent these two agents are in this function. The question of the role of vitamin D in renal tubular reabsorption of phosphate remains unsettled, although recently there has been some striking evidence presented that both vitamin D and parathyroid hormone may be involved in the phosphaturic response of the kidney (Bonjour & Fleisch, 1977). This wdl not be discussed further except to state that this mechanism involves phosphate excretion when serum phosphate concentration is either normal or above normal and apparently does not operate under conditions of even slight hypophosphataemia. During the course of the past decade and a half, it has been demonstrated that vitamin D must undergo the metabolic conversions shown in Fig. 2 before it can carry out these functions (DeLuca, 1974; Omdahl & DeLuca, 1973; DeLuca & Schnoes, 1976). As a reminder, it is known that in the skin there exists an abundant supply of 7dehydrocholesterol (Idler & Baumann, 1952), which is photolysed by ultraviolet light to yield vitamin D3 which has recently been isolated from skin and identified (R. Esvelt & H.F. DeLuca, unpublished results). Because man does not produce sufficient amounts of vitamin D by this mechanism, he relies largely on dietary sources. Vitamin D produced either in skin or absorbed from the small intestine is rapidly accumulated in the liver (Ponchon et al., 1969, Olson et al., 1976) where it undergoes its first obligatory reaction in which it is converted to the major circulating form of vitamin D, 25-hydroxyvitamin D3 (25-OHD3) (Ponchon er al., 1969; Horsting & DeLuca, 1969; Bhattacharyya & DeLuca, 1974; Olson et of., 1976). The exact cellular site of this hydroxylation remains unknown although it is known that the reaction requires NADPH, molecular oxygen and magnesium ions (Bhattacharyya & DeLuca, 1974). Although the product 25-OHD3 does suppress the 25hydroxylase, it is well known that when large amounts of substrate are provided the block in the hydroxylase can be overcome. Thus the administration of large amounts of vitamin D can bring about high levels of circulating 25-OHD3 (Blunt et al., 1968; Haddad & Stamp, 1974). The 25-hydroxylase is not blocked by cytochrome P-450 inhibitors nor is it stimulated by phenobarbitone and phenytoin administration (Horsting, 1970). Thus it does not appear
Fig. 2. The metabolism of vitamin D required for function.
4s
H. F. DeLuca
a t this stage to be a mixed-function oxygenase. 25-OHD3is transported on a 52,000 mol. wt. a-globulin (Botham et al., 1976; Haddad & Wdgate, 1976; Imawari et af., 1976) which appears to be identical with the group specific protein isolated previously. T h s transport protein also transports other metabolites of vitamin D and vitamin D itself although it prefers 25-OHD3. The 25-OHD3 at physiological concentrations apparently does not act directly in any known physiological process. Instead it must be further activated before it can function in intestine, bone and kidney. T h s further activation occurs in the ludney exclusively (Fraser & Kodicek, 1970; Gray et al., 1971) and specifically in the mitochondria (Gray er al., 1972), which converts 250HD3 to the active hormone, 1,25dihydroxyvitamin D3 (1,25
VITAMIN D METABOLISM H E C T O R F. D E L U C A Department of Biochemishy, College o f Agricultural Life Sciences, University of Wisconsin-Madison,Madison, Wisconsin, U.S.A.
SUMMARY
During the past decade, an explosion of information has become avdable on the metabolism and function of vitamin D w h c h is of great importance to clinicians in the treatment of metabolic bone disease. We have learned that vltamin D is the precursor of at least one hormone and that t h s hormone carries out functions in calcium and phosphorus metabolism brmging about mineralization of bone on one hand, and the prevention of hypocalcaemic tetany on the other. It may also function in the prevention of such degenerative bone diseases as osteoporosis. An important analogue of this hormone, la-hydroxyvitamin D3 has been prepared and is used successfully in the treatment of a variety of clinical conditions. This presentation wdl summarize these findings and their possible implications in these metabolic bone diseases. I t is well known that vitamin D brings about the mineralization of organic matrix elaborated by both chondrocytes and osteoblasts (Sebrell & Harris. 1954). In so doing, it prevents the disease, rickets, in the young and osteomalacia in the adult. It also functions in the prevention of hypocalcaemic tetany, a very critical function which it shares with the parathyroid hormone (DeLuca, 1974; Rasmussen et al:. 1963; Harrison & Harrison. 1964). Because vitamin D is essential for intestinal calcium absorption and because its presence is necessary for the intestine t o adapt its efficiency of calcium absorption to the calcium needs of the organism, vitamin D plays a role in the prevention of bone loss or osteoporosis (DeLuca, 1976, 1977). Finally, it is known that vitamin D improves muscle strength, although the mechanism of this function is not understood even at the physiological level and remains to be studied. Figure 1 summarizes the sites at which vitamin D is known t o function in the prevention of the above described pathological conditions. The failure of bone mineralization in vitamin D deficiency is for the most part the result of an insufficient supply of calcium and phosphorus to the calcification centres (DeLuca, 1967). Although there is some sentiment that vitamin D must function directly in the mineralization process, so far a clear experimental demonstration of this possibility has not appeared. We, therefore, leave open the Correspondence: Dr H . F. DeLuca. Department of Biochemistry, 420 Henry Mall, University of Wisconsin-Madison, 'Cladison, Wisconsin 53706, U.S.A. IS
2s
H. F. DeLuca
distinct possibility that some form of vitamin D may function in that process. To prevent hypocalcaemic tetany and to provide sufficient amounts of calcium and phosphorus for minerahzation, it is well known that vitamin D functions to elevate both plasma calcium and plasma phosphate concentrations. To provide the necessary calcium and phosphorus, vitamin D stimulates at least three transport systems; the best known is the system which transports calcium across intestinal wall against an electro-chemical potential gradient (Nicolaysen & Eeg-Larsen, 1953; Schachter & Rosen, 1959; Wasserman, 1963; OmdaN & DeLuca, 1973). This process is stimulated primarily by vitamin D and indirectly by the parathyroid hormone as will be discussed later. Phosphate is the normal accompanying anion but in addition to this process, vitamin D initiates a phosphate transport reaction which is easily demonstrable in the distal portions of the small intestine (Harrison & Harrison, 1961 ; Kowarski & Schachter, 1969; Chen er af., 1974). Phosphate is transported against an electrochemical potential gradient (Walling, 1977), in a sodium dependent (Taylor, 1974) and calcium independent process (Harrison & Harrison, 1961 ; Kowarslu & Schachter, 1969; Chen et af., 1974). Thus there are two mechanisms in the small intestine whch contribute plasma calcium and phosphate to the extracellular fluid compartment. An extremely important source of calcium and phosphorus is previously mineralized bone. It must be recognized that the mineral phase of bone is separated from the extracellular fluid compartment by a membrane of cells, usually believed to be the osteoblastic and osteoclastic membrane barriers (Holtrop & Weinger, 1972; Talmage er al., 1975). I t is believed that vitamin D together with the parathyroid hormone stimulates the transfer of calcium from the bone fluid compartment to the extracellular fluid compartment (Rasmussen er al., 1963; Garabedian et al., 1974). At physiological concentrations both agents are required b u t the amount of each required can be diminished by a large excess of the other (Rasmussen ef al., 1963). This process is probably extremely important in the minute-to-minute control of plasma calcium concentration.
b d PI
Fig. 1. Physiological sites of action of 1,2540H),D, and the parathyroid hormone in their regulation of calcium and phosphorus metablism.
Vitamin D metabolism
3s
Another site of vitamin D action is the kidney. It has recently been demonstrated that vitamin D does improve renal reabsorption of calcium (Steele er al., 1975) and it is well known that the parathyroid hormone also improves renal reabsorption of calcium (Kleeman er al., 1961). It is not known how interdependent these two agents are in this function. The question of the role of vitamin D in renal tubular reabsorption of phosphate remains unsettled, although recently there has been some striking evidence presented that both vitamin D and parathyroid hormone may be involved in the phosphaturic response of the kidney (Bonjour & Fleisch, 1977). This wdl not be discussed further except to state that this mechanism involves phosphate excretion when serum phosphate concentration is either normal or above normal and apparently does not operate under conditions of even slight hypophosphataemia. During the course of the past decade and a half, it has been demonstrated that vitamin D must undergo the metabolic conversions shown in Fig. 2 before it can carry out these functions (DeLuca, 1974; Omdahl & DeLuca, 1973; DeLuca & Schnoes, 1976). As a reminder, it is known that in the skin there exists an abundant supply of 7dehydrocholesterol (Idler & Baumann, 1952), which is photolysed by ultraviolet light to yield vitamin D3 which has recently been isolated from skin and identified (R. Esvelt & H.F. DeLuca, unpublished results). Because man does not produce sufficient amounts of vitamin D by this mechanism, he relies largely on dietary sources. Vitamin D produced either in skin or absorbed from the small intestine is rapidly accumulated in the liver (Ponchon et al., 1969, Olson et al., 1976) where it undergoes its first obligatory reaction in which it is converted to the major circulating form of vitamin D, 25-hydroxyvitamin D3 (25-OHD3) (Ponchon er al., 1969; Horsting & DeLuca, 1969; Bhattacharyya & DeLuca, 1974; Olson et of., 1976). The exact cellular site of this hydroxylation remains unknown although it is known that the reaction requires NADPH, molecular oxygen and magnesium ions (Bhattacharyya & DeLuca, 1974). Although the product 25-OHD3 does suppress the 25hydroxylase, it is well known that when large amounts of substrate are provided the block in the hydroxylase can be overcome. Thus the administration of large amounts of vitamin D can bring about high levels of circulating 25-OHD3 (Blunt et al., 1968; Haddad & Stamp, 1974). The 25-hydroxylase is not blocked by cytochrome P-450 inhibitors nor is it stimulated by phenobarbitone and phenytoin administration (Horsting, 1970). Thus it does not appear
Fig. 2. The metabolism of vitamin D required for function.
4s
H. F. DeLuca
a t this stage to be a mixed-function oxygenase. 25-OHD3is transported on a 52,000 mol. wt. a-globulin (Botham et al., 1976; Haddad & Wdgate, 1976; Imawari et af., 1976) which appears to be identical with the group specific protein isolated previously. T h s transport protein also transports other metabolites of vitamin D and vitamin D itself although it prefers 25-OHD3. The 25-OHD3 at physiological concentrations apparently does not act directly in any known physiological process. Instead it must be further activated before it can function in intestine, bone and kidney. T h s further activation occurs in the ludney exclusively (Fraser & Kodicek, 1970; Gray et al., 1971) and specifically in the mitochondria (Gray er al., 1972), which converts 250HD3 to the active hormone, 1,25dihydroxyvitamin D3 (1,25
