Metabolism of 1alpha-hydroxyvitamin D3 by cytochrome P450scc to biologically active 1alpha,20-dihydroxyvitamin D3

Cytochrome P450scc (CYP11A1) metabolizes vitamin D3 to 20-hydroxyvitamin D3 as the major product, with subsequent production of dihydroxy and trihydroxy derivatives. The aim of this study was to determine whether cytochrome P450scc could metabolize 1alpha-hydroxyvitamin D3 and whether products were biologically active. The major product of 1alpha-hydroxyvitamin D3 metabolism by P450scc was identified by mass spectrometry and NMR as 1alpha,20-dihydroxyvitamin D3. Mass spectrometry of minor metabolites revealed the production of another dihydroxyvitamin D3 derivative, two trihydroxy-metabolites made via 1alpha,20-dihydroxyvitamin D3 and a tetrahydroxyvitamin D3 derivative. The Km for 1alpha-hydroxyvitamin D3 determined for P450scc incorporated into phospholipid vesicles was 1.4 mol substrate/mol phospholipid, half that observed for vitamin D3. The kcat was 3.0 mol/min/mol P450scc, 6-fold lower than that for vitamin D3. 1alpha,20-Dihydroxyvitamin D3 inhibited DNA synthesis by human epidermal HaCaT keratinocytes propagated in culture, in a time- and dose-dependent fashion, with a potency similar to that of 1alpha,25-dihydroxyvitamin D3. 1alpha,20-Dihydroxyvitamin D3 (10 microM) enhanced CYP24 mRNA levels in HaCaT keratinocytes but the potency was much lower than that reported for 1alpha,25-dihydroxyvitamin D3. We conclude that the presence of the 1-hydroxyl group in vitamin D3 does not alter the major site of hydroxylation by P450scc which, as for vitamin D3, is at C20. The major product, 1alpha,20-dihydroxyvitamin D3, displays biological activity on keratinocytes and therefore might be useful pharmacologically.

UVB-induced conversion of 7-dehydrocholesterol to 1alpha,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model

We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D3 to the hormone calcitriol (1alpha,25-dihydroxyvitamin D3). This study was initiated to test whether the ultraviolet-B-induced photolysis of provitamin D3 (7-dehydrocholesterol), which results in the formation of vitamin D3, can generate calcitriol in an in vivo-like human skin equivalent model made of fibroblasts in a collagen matrix as the dermal component and keratinocytes as the epidermal component. Cultures were preincubated with increasing concentrations of 7-dehydrocholesterol (0.53-5.94 nmol per cm2 human skin equivalent) at 37 degrees C and irradiated with monochromatic ultraviolet B at wavelengths ranging from 285 to 315 nm (effective ultraviolet doses 7.5-45 mJ per cm2). In our in vitro model irradiation with ultraviolet B resulted in a sequential metabolic process with generation of previtamin D3 followed by the time-dependent formation of vitamin D3, 25-hydroxyvitamin D3, and ultimately calcitriol in the femtomolar range. Unirradiated cultures and irradiated cultures without keratinocytes generated no calcitriol. Irradiation of skin equivalents at wavelengths > 315 nm generated no or only trace amounts of calcitriol. The ultraviolet-B-triggered conversion of 7-dehydrocholesterol to calcitriol was strongly inhibited by ketoconazole indicating the involvement of P450 mixed function oxidases. The amount of calcitriol generated was dependent on the 7-dehydrocholesterol concentration, on wavelength, and on ultraviolet B dose. Hence, keratinocytes in the presence of physiologic concentrations of 7-dehydrocholesterol and irradiated with therapeutic doses of ultraviolet B may be a potential source of biologically active calcitriol within the epidermis.

Synthesis, stereochemistry, and biological activity of 1alpha,23,25-trihydroxy-24-oxovitamin D3, a major natural metabolite of 1alpha,25-dihydroxyvitamin D3

The C(23) epimers of 1alpha,23,25(OH)3-24-oxovitamin D3, a major natural metabolite of the secosteroid hormone, 1alpha,25(OH)2D3, were chemically synthesized for the first time. The metabolite was synthesized by palladium coupling of the appropriate CD ring analog with an A ring enyne. Various approaches from quinic acid to the A ring precursors were explored, and a new route to the A ring enyne from quinic acid was developed. The C(23) stereochemistry of the natural 1alpha,23,25(OH)3-24-oxovitamin D3 produced in neonatal human keratinocytes was determined to be S on the basis of the 1H NMR and the HPLC data. The biological activity of 1alpha,23(S), 25(OH)3-24-oxovitamin D3 in primary cultures of bovine parathyroid cells was determined by comparing the potency of this metabolite to that of 1alpha,25(OH)2D3 in suppression of parathyroid hormone (PTH) secretion. The results indicate that 1alpha,23(S), 25(OH)3-24-oxovitamin D3 potently suppressed PTH secretion even at concentrations as low as 10(-)12 M and is equipotent with 1alpha, 25(OH)2D3. The high activity of 1alpha,23(S),25(OH)3-24-oxovitamin D3 cannot be explained on the basis of its affinity for the vitamin D receptor as this metabolite was found to be 10 times less effective than radioinert 1alpha,25(OH)2D3 in blocking the uptake and receptor binding of [3H]-1alpha,25(OH)2D3 in intact parathyroid cells. Further studies are required to explain the molecular basis for the activity of 1alpha,23(S),25(OH)3-24-oxovitamin D3 in its ability to suppress PTH secretion. In summary, our present study indicates that the C(23) stereochemistry of the natural 1alpha,23, 25(OH)3-24-oxovitamin D3 is S and this metabolite is equipotent to 1alpha,25(OH)2D3 in suppressing PTH secretion.

Synthesis of active metabolite(s) from 1 alpha-hydroxyvitamin D3 by human monocytic leukemia cells

Synthesis of the biologically active metabolite(s) from 1 alpha-hydroxyvitamin D3 (1 alpha(OH)D3) was examined in various types of human leukemia cell lines. Untreated monocytoid leukemia cells (U937 and HEL/S) metabolized 1 alpha (OH)D3 to the active metabolite(s), possibly 1 alpha, 24- and/or 1 alpha, 25-dihydroxyvitamin D3, and these cells were efficiently induced to differentiate by treatment with 1 alpha (OH)D3. However, the other types of leukemia cells did not efficiently metabolize it and were not induced to differentiate by 1 alpha (OH)D3. The possible therapeutic advantage of 1 alpha (OH)D3 in the treatment of monocytic leukemia is discussed.

Regulation of 1,25-dihydroxyvitamin D production in human keratinocytes by interferon-gamma

Interferon-gamma (IFN gamma) and 1,25-dihydroxyvitamin D [1,25-(OH)2D] each have potent antiproliferative and prodifferentiating effects on keratinocytes. Since keratinocytes produce 1,25-(OH)2D, we explored the possibility that IFN gamma acted on keratinocytes in part by regulating 1,25-(OH)2D production. We cultured human neonatal foreskin keratinocytes for various periods of time in the presence of various concentrations of IFN gamma before assessing their ability to produce 1,25-(OH)2D. The production of 1,25-(OH)2D by preconfluent keratinocytes grown in the presence of serum (which retards differentiation) was stimulated by 1.8 nM IFN gamma. Postconfluent keratinocytes did not respond to 1.8 nM IFN gamma. The production of 1,25-(OH)2D by keratinocytes grown in serum-free medium was maximally stimulated by 0.006 nM IFN gamma and inhibited at concentrations greater than 0.06 nM. Keratinocytes grown in 0.1 mM calcium serum-free medium (which prevents differentiation) were more sensitive to both the stimulatory and inhibitory effects of IFN gamma than keratinocytes grown in 1.2 mM calcium serum-free medium (which permits differentiation). The stimulatory effect of IFN gamma on 1,25-(OH)2D production was maximal after 2 days of incubation. Incubations longer than 2 days showed increasingly less stimulation at the low IFN gamma concentrations and increasingly greater inhibition at the higher IFN gamma concentrations. The inhibitory effects of IFN gamma on 1,25-(OH)2D production paralleled the inhibitory effects of IFN gamma on cell growth. Thus, IFN gamma does regulate 1,25-(OH)2D production by keratinocytes. However, this regulation is modulated by the state of keratinocyte proliferation and differentiation and is influenced by calcium and undefined factors in serum. The data are consistent with the possibility that IFN gamma alters keratinocyte differentiation in part by regulating 1,25-(OH)2D production.

Effect of indomethacin on plasma levels of vitamin D metabolites, oestradiol and progesterone in rabbits during early pregnancy

1. Inhibition of prostaglandin (PG) synthesis by indomethacin (Id) during early pregnancy in rabbits apparently disrupts the process of sex steroid production by the ovaries. 2. The role of PGs as mediators in steroidogenesis was tested by investigating the effect of Id alone or in combination with progesterone, with oestradiol and progesterone, or with a mixture of PGs, on plasma levels of 25-hydroxyvitamin D (25(OH)D) and 1,25-dihydroxyvitamin D (1,25(OH)2D) in rabbits at an early stage of pregnancy. 3. Id alone significantly reduced plasma levels of both 25(OH) D and 1,25(OH)2D. Treatment with Id in combination with either oestradiol and progesterone, or with a mixture of PGs, resulted in the restoration of plasma levels of both vitamin D metabolites as well as the restoration of plasma levels of progesterone, to their control values.

Modulation of renal production of 24,25- and 1,25-dihydroxyvitamin D3 in young and adult rats by dietary calcium, phosphorus, and 1,25-dihydroxyvitamin D3

The purpose of this study was to examine the effects of dietary calcium (Ca), phosphorus (P), and vitamin D3 metabolites on the renal metabolism of 25-hydroxyvitamin D3 (25OHD3) to either 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] or 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] in the rat. The regulation of 25OHD3 metabolism was studied in both young and adult rats, since previous studies have suggested a change in the renal metabolism of 25OHD3 with age. Renal 25OHD3 metabolism was measured in vitro by incubating renal cortical slices with tritiated 25OHD3 and quantifying tritiated metabolites by high pressure liquid chromatography. The apparent Michaelis constant for the conversion of 25OHD3 to 1,25-(OH)2D3 in this system was 1.16 microM. Experiments were conducted in rats fed a vitamin D-deficient diet containing either 0.02% Ca (low Ca) or 1.20% Ca (high Ca) for 4 weeks. Young rats (4 weeks old) fed the low Ca diet demonstrated a 2.8-fold increase in 1,25-(OH)2D3 production, but no change in 24,25-(OH)2D3 production compared to young rats fed the high Ca diet. Adult rats (12 months old) fed the low Ca diet showed no change in 1,25-(OH)2D3 production, but exhibited a decrease in 24,25-(OH)2D3 production compared to adult rats fed the high Ca diet. Repletion of the young rats fed the low Ca diet with 1,25(OH)2D3 resulted in a marked decrease in 1,25-(OH)2D3 production and an increase in 24,25-(OH)2D3 production. Repletion of the adult rat resulted in no change in 1,25-(OH)2D3 production, but a significant increase in 24,25-(OH)2D3 production. When young rats were fed diets containing various levels of Ca and P, it was found that 1,25-(OH)2D3 production was inversely correlated with plasma Ca over the range 4--13 mg/dl. Since the plasma Ca level of the adult rat was 11-12 mg/dl regardless of diet, this high concentration may explain the lack of 1,25-(OH)2D3 production observed in the adult.

Biosynthesis, purification and receptor binding properties of high specific radioactivity 1 alpha, 24(R),25-trihydroxy-[26,27-methyl-3H]-vitamin D3

A procedure for the biosynthesis and purification of 1 alpha, 24(R),25-trihydroxy[26,27-methyl-3H]-vitamin D3 (1,24,25-(OH)3[3H]D3) is reported. A kidney homogenate from chicks receiving a high calcium diet (3%) and oral supplements of 1 alpha, 25-dihydroxyvitamin D3 (1,25-(OH)2D3) was used for C-24-hydroxylation of 1 alpha, 25-dihydroxy[26,27-methyl-3H]-vitamin D3 (1,25-(OH)2[3H]D3), in vitro. Extraction and purification of the homogenate lipid fraction by Sephadex LH-20 and high performance liquid chromatography yielded radiochemically pure 1,24-25-(OH)3[3H]D3 with a specific radioactivity equivalent to the initial substrate (166 Ci/mmol). The authenticity of the generated metabolite was assessed by co-migration with synthetic 1,24,25-(OH)3D3 on high performance liquid chromatography and by equimolar competition with authentic radioinert 1,24,25-(OH)3D3 for binding to a purified receptor protein from rat kidney. Binding studies indicate the trihydroxylated metabolite competes 40-50% as effectively as 1,25-(OH)2D3 for hormone binding sites. Further analysis of 1,24,25-(OH)3D3-receptor interaction reveals a high-affinity, saturable binding with an apparent K4 of 2.2 x 10(-9) M. These studies demonstrate that although slightly less active than 1,25-(OH)2D3, 1,24,25-(OH)3D3 is capable of hormone-like interactions, in vitro. The availability of this high specific radioactivity sterol should allow for clarification of its potential physiologic significance.

Uptake and 25-hydroxylation of vitamin D3 by isolated rat liver cells

The physiological roles played by hepatocytes and nonparenchymal cells of rat liver in the metabolism of vitamin D3 have been investigated. Tritium-labeled vitamin D3 dissolved in ethanol was administered intravenously to two rats. Isolation of the liver cells 30 and 70 min after the injection showed that vitamin D3 had been taken up both by the hepatocytes and by the nonparenchymal liver cells. The relative proportion of vitamin D3 that accumulated in the nonparenchymal cells increased with time. Perfusion of the isolated rat liver with [3H] vitamin D3 added to the perfusate confirmed the ability of both cell types to efficiently take up vitamin D3 from the circulation. By a method based on high pressure liquid chromatography and isotope dilution-mass fragmentography it was found that isolated liver cells in suspension had a considerable capacity to take up vitamin D3 from the medium. About 2.5 fmol of vitamin D3 were found to be associated with each hepatocyte or nonparenchymal cell after 1 h of incubation. 25-Hydroxylation in vitro was found to be carried out only by the hepatocytes. The rate of hydroxylation was about the same whether the cells were isolated from normal or rachitic rats (3.5 and 4 pmol of 25-hydroxyvitamin D3 formed per h per 10(6) cells, respectively). The possibility that the nonparenchymal cells might serve as a storage site for vitamin D3 in the liver is discussed.

1 Alpha-25,26-trihydroxyvitamin D3: an in vivo and in vitro metabolite of vitamin D3

A new metabolite of vitamin D3 has been isolated from the plasma of vitamin D3 treated cows and has been generated from 25(S),26-dihydroxyvitamin D3 with homogenates of vitamin D deficient chick kidney. This metabolite has been identified as 1,25,26-trihydroxyvitamin D3 by comigration with synthetic 1,25(S),26-trihydroxyvitamin D3 in four chromatographic systems, ultraviolet spectroscopy, mass spectrometry, and high-pressure liquid chromatography and mass spectrometry of derivatives. 1,25(S),26-Trihydroxyvitamin D3 is one-tenth as effective as 1,25-dihydroxyvitamin D3 in binding to the chick intestinal cytosol 1,25-dihydroxyvitamin D receptor. Either 25(S),26-dihydroxyvitamin D3 or 1,25-dihydroxyvitamin D3 can serve as precursor for in vitro production of 1,25,26-trihydroxyvitamin D3 by chick kidney tissue.

Human bone cells in culture metabolize 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3

Bone cells respond to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) for mineral mobilization and contain receptors for 1,25(OH)2D3. We report here the expression of 25-hydroxyvitamin D3 (25 (OH)D3) metabolizing enzymes in primary cultures of human bone cells, as well as n a human osteosarcoma cell line. Human bone cells were obtained by enzyme digestion of the extracellular matrix of bone from iliac crest biopsies from 3 male patients without primary bone disease. These cells were plated (5 X 10(4)/min) in medium with 10% fetal calf serum and proliferated to confluence in 10-14 days. At confluence, the medium was replaced with serum-free medium. The cells were preincubated in this serum-free medium for 24 h prior to incubating them 2-4 h with [3H]25(OH)D3 (10-20 nM). The vitamin D metabolites synthesized during this incubation were extracted from the medium and cells with dichloromethane, then separated by chromatography on Sephadex LH-20, followed by high performance liquid chromatography. The cells synthesized 1,25(OH)2D3 and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) with the specific activities of the 1- and 24-hydroxylases similar in magnitude to those in kidney cells in vitro. The enzymes could be regulated by external perturbations, in that the activity of the 1-hydroxylase was inhibited by preincubation of the cells for 8 h with 1,25(OH)2D3 (10 nM), whereas the 24-hydroxylase was enhanced. Incubation of the cells in a low calcium medium (0.6 mM) depressed the 24-hydroxylase activity. We conclude: 1) normal human bone cells can produce 1,25(OH)2D3 and 24,25(OH)2D3 in vitro in amounts similar to kidney cells, suggesting a physiological significance and 2) this synthesis could account for the increase in osteoclast number in anephric patients with renal osteodystrophy.

23,25-Dihydroxyvitamin D3: a natural precursor in the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone

To elucidate the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone, two known metabolites of 25-hydroxyvitamin D3--23,25-dihydroxyvitamin D3 and 25,26-dihydroxyvitamin D3--were incubated individually with kidney homogenate prepared from vitamin D-supplemented chickens, a preparation known to produce the lactone from 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3-26,23-lactone produced in vitro was then separated, purified, identified, and quantitated by consecutive straight-phase and reverse-phase high-performance liquid chromatography. 23,25-Dihydroxyvitamin D3 is a far better substrate for production of 25-hydroxyvitamin D3-26,23-lactone than is 25,26-dihydroxyvitamin D3. Production of lactone is highly selective for the natural 23(S)-hydroxy-23,25-dihydroxyvitamin D3 while both epimers of 25,26-dihydroxyvitamin D3 resulted in small amounts of product comigrating with natural lactone. It appears that 23(S),25-dihydroxyvitamin D3, but not 25,26-dihydroxyvitamin D3, is a natural precursor in the synthesis of 25-hydroxyvitamin D3-26,23-lactone; this result also implies that the configuration of the lactone at C-23 is S.

The synthesis of dihydroxylated vitamin D metabolites by human renal mitochondria

Mitochondrial preparations from 4 human kidneys produced 1,25-dihydroxycholecalciferol and 24,25 dihydroxycholecalciferol at rates of 0.019 - 0.114 and 0.029 - 0.164 pmol/mg/min respectively at a 25-hydroxycholecalciferol concentration of 1 mumol per litre. Mitochondria from a fifth kidney failed to produce either metabolite.

Conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in renal slices from the rat

Isolated renal cortical slices were used to study the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25](OH2)D3] by the rat kidney. Production of 1,25-(OH)2D3 and 24,25-(OH)2D3 was linear with time (30-90 min) and tissue weight (40-250 mg). Production of 1,25-(OH)2D3 was greatest (134 +/- 17 pg/mg tissue.h) in animals fed a low calcium, vitamin D-deficient diet. The greatest 24,25-(OH)2D3 production (106 +/- 17 pg/mg tissue.h) was seen in animals fed a high calcium, vitamin D-replete diet, 1,25-(OH)2D3 production was reduced to 23% of maximum by the addition of 1.2% calcium or 0.8% strontium to the vitamin D-deficient, low calcium diet. Production of 1,25-(OH)2D3 and 24,25-(OH)2D3 was greatly reduced in renal cortical slices that had been heated before incubation. Slices of renal medulla produced only small amounts of 1,25-(OH)2D3 compared to slices of renal cortex. These studies provide direct evidence for the production of 1,25-(OH)2D3 and 24,25-(OH)2D3 by the mammalian renal cortex. They also demonstrate that this production may be modulated by dietary calcium, strontium, and vitamin D.

Insulin permits parathyroid hormone stimulation of 1,25-dihydroxyvitamin D3 production in cultured kidney cells

Primary cultures of chick kidney cells in serum free medium respond to PTH with increased production of 1,25(OH)2D3 only when exposed to insulin. The response of 1,25(OH)2D3 is maximal at 5 ng bPTH (1-34) per ml and decreases at higher hormone concentrations. Increased 1,25(OH)2D3 synthesis is not evident after 30 minutes exposure to bPTH and is maximal at 4-6 hours of treatment. Insulin does not increase the cyclic AMP response to PTH suggesting that whatever permissive role it is playing occurs beyond the generation of cyclic AMP.

Stimulation of 1,25-dihydroxycholecalciferol production by prolactin and related peptides in intact renal cell preparations in vitro

Ovine prolactin stimulated the 1 alpha-hydroxylase activity in isolated renal tubules and especially in primary kidney cell cultures, both prepared from vitamin D-deficient chicks. In primary chick kidney cell cultures, treated for 48 h with 1,25-dihydroxycholecalciferol (to induce the 24-hydroxylase activity) ovine prolactin, after a 1 h incubation period, stimulated the 1 alpha-hydroxylase activity without affecting the 24-hydroxylase activity. Similar results were obtained with related peptide hormones such as human growth hormone, chicken growth hormone, and human placental lactogen. These observations are discussed in relation to the possible role of these peptide hormones as modulators of 1,25-dihydroxycholecalciferol production in physiological situations of calcium stress, such as pregnancy, lactation, growth in mammals and egg laying in birds.

In vitro synthesis of 1 alpha,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol by isolated calvarial cells

The question of whether the skeleton metabolizes 25-hydroxycholecalciferol [25(OH)D3] to more-polar products was studied. Calvarial cells were dispersed from 16-day old chicken embryos by using collagenase and then grown in culture in serum-free medium. Confluent cell cultures were incubated with 7 nM 25(OH)[3H]D3 for 2 hr, and the vitamin D metabolites were then extracted. At least four polar metabolites were produced. Based on separation by Sephadex LH-20 chromatography followed by high-pressure liquid chromatography, two of these metabolites were identified as 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3]. These metabolites were also produced by cultured kidney cells but not by liver, heart muscle, or skin cells isolated from the same embryos. The specific activities of the calvarial 1- and 24-hydroxylases were similar in magnitude to those in isolated kidney cells. The specific activity of the calvarial 25(OH)D3:1-hydroxylase was inhibited by an 8-hr preincubation with 1,25(OH)2D3, whereas the 24-hydroxylase was enhanced. It is concluded that (i) vitamin D metabolism by isolated cells is organ-specific, (ii) calvarial cells produce active metabolites of vitamin D in significant amounts, (iii) vitamin D metabolism by calvarial cells is regulated by 1,25(OH)2D3, and (iv) locally produced, active metabolites could act locally, thereby adding a new dimension to the regulation of mineral metabolism by vitamin D metabolites.

The photoproduction of 1 alpha,25-dihydroxyvitamin D3 in skin: an approach to the therapy of vitamin-D-resistant syndromes

Cutaneous 7-dehydrocholesterol, exposed to ultraviolet radiation, converts to previtamin D3, which in turn converts in skin to vitamin D3 and is carried into the circulation. We investigated the feasibility of the photochemical conversion in skin of hydroxylated derivatives of 7-dehydrocholesterol - such as 1 alpha, 25-dihydroxy-7-dehydrocholesterol (1 alpha, 25-(OH)2-7-DHC) - to the corresponding hydroxylated previtamin as an alternative method of delivery of 1 alpha, 25-dihydroxyvitamin D3 (1 alpha, 25-(OH)2-D3) to subjects who are deficient in the endogenous metabolite. In human volunteers and in vitamin-D-deficient rats [24-3H] 1 alpha, 25-(OH)2-D3 appeared in blood after [24-3H] 1 alpha, 25-(OH)2-7-DHC was applied to the skin and exposed to ultraviolet radiation. In anephric rats, intestinal calcium absorption and serum calcium levels were elevated after a topical dose of 1 alpha, 25-(OH)2-7-DHC combined with ultraviolet phototherapy. Delivery of equivalent doses of 1 alpha, 25-(OH)2-D3 through the skin and orally showed that there was more prolonged stimulation in intestinal calcium absorption and serum calcium elevation after cutaneous administration. The photochemical conversion of precursors may be useful in the treatment of patients with impaired vitamin-D metabolism.

Prolactin but not growth hormone stimulates 1,25-dihydroxyvitamin D3 production by chick renal preparations in vitro

We studied the effect of PRL from two species (bovine and turkey) and GH from two species (bovine and turkey) on 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] production by two whole cell preparations from vitamin D-deficient chick kidneys (slices and tubules). We observed that 8 ng/ml turkey PRL stimulated 1,25(OH)2D3 production by renal tubules and slices. Ovine PRL had a similar effect on 1,25(OH)2D3 production but at higher concentrations. In contrast, neither bovine GH nor turkey GH stimulated 1,25(OH)2D3 production appreciably at doses up to 1000 ng/ml. The effect of PRL on 1,25(OH)2D3 production by renal tubules required a 3-h preincubation, although its effect on 1,25(OH)2D3 production by renal slices was immediate. We conclude that PRL, but not GH, directly stimulates 1,25(OH)2D3 production by the chick kidney.

Stimulation of 1,25-dihydroxyvitamin D production by parathyroid hormone and hypocalcemia in man

The serum levels of 1.25-dihydroxycholecalciferol [1,25(OH)2D3] were increased in five patients with primary hyperparathyroidism [60 +/- 13 (SD) pg/ml; normal value, 33 +/- 15 (SD) pg/ml] but fell rapidly after parathyroidectomy to values of 23 +/- 9 (SD) pg/ml. This was accompanied by parallel decreases in the serum concentrations of calcium and immunoreactive parathyroid hormone. Over the following 5--35 days, the serum 1,25(OH)2D3 concentrations increased markedly to levels of 59 +/- 17 (SD) pg/ml, which could most likely be explained by a stimulatory effect of the hypocalcemia per se on the renal production of 1,25(OH)2D3.

Hepatic metabolism of vitamin D3 in streptozotocin-induced diabetic rat

The effect of streptozotocin-induced diabetes on the in vitro conversion of vitamin D3 to 25-hydroxy-vitamin D3 (25-OHD3) by isolated liver microsomes from rachitic rats was examined. Enzymic activity was significantly less than that observed in control animals (P less than 0.001). Administration of insulin restored activity almost to control values. These findings provide evidence that diabetes in this animal model produces alterations in the metabolism of vitamin D.

Assessment of plasma 25-hydroxyvitamin D response to ultraviolet irradiation over a controlled area in young and elderly subjects

1. The response of 25-hydroxyvitamin D [25-(OH)D] to artificial ultraviolet irradiation applied to a known area of dorsal skin was investigated in 18 subjects, small quantities of ultraviolet energy being used. Ultraviolet irradiation was administered on days, 1, 3, 5, 8, 10, 12, 15 and 17, a total of 15 min being delivered over this time. In 15 subjects plasma 25-(OH)D showed a significant increase after a total of 15 min exposure but three subjects failed to demonstrate any increment. Plasma 25-(OH)D did not increase in any subject after 2·5 min of ultraviolet irradiation (irradiation on days 1 and 3).

2. Responses were compared in young and old, in male and female and in normal and osteomalacic subjects. No significant difference in response was found between these groups.

3. When plasma volume was taken into account, it was possible to calculate the increase in nmol of plasma 25-(OH)D/cm2 skin irradiated. This was 0·024 nmol/cm2 with no sex difference, over the 17 days of irradiation.

4. Exposure to ultraviolet irradiation over a small area of dorsal skin led to a rapid rise of plasma 25-(OH)D in most subjects with a subsequent plateau in subjects studied for up to 15 min total exposure. This contrasted with the prolonged increase in plasma 25-(OH)D continuing over several weeks in response to whole-body ultraviolet irradiation. This may indicate that cutaneous synthesis of vitamin D is rapid but limited, and that the considerable rise in plasma 25-(OH)D during whole-body irradiation may originate from vitamin D synthesized during the first few exposures.

Salmon calcitonin-induced stimulation of 1 alpha,25-dihydroxycholecalciferol synthesis in rats involving a mechanism independent of adenosine 3':5'-cyclic monophosphate

The effect of natural salmon calcitonin on accumulation in plasma of 1 alpha,25-dihydroxy-[3H]cholecalciferol from 25-hydroxy[3H]cholecalciferol in vivo was investigated in vitamin D-deficient thyroparathyroidectomized rats into which graded doses of the hormone were continuously infused by use of a balance study system. A dose-dependent increase in plasma concentrations of 1 alpha,25-dihydroxy[3H]cholecalciferol was observed with calcitonin infusion for 6--30h at a rate greater than 20 M.R.C. m-units/h. Infusion of parathyrin or cyclic AMP produced a similar stimulation [Horiuchi, Suda, Takahashi, Shimazawa & Ogata (1977) Endocrinoly 101, 969--974], but the maximal effect of calcitonin was additive to that of either parathyrin or cyclic AMP. Furthermore concurrent infusion of theophylline (0.5 mumol/h) did not potentiate the effect of submaximal doses (3 and 20 M.R.C. m-units/h) of calcitonin. Plasma concentrations of calcium showed a decrease with calcitonin infusion for 30h, but those of Pi remained unchanged. These results strongly suggest that the rat kidney is endowed with a calcitonin-sensitive 1 alpha-hydroxylase system that is separate from the parathyrin/cyclic AMP system and is independent of changes in plasma Pi.

In vitro production of 1,25-dihydroxyvitamin D3 by rat placental tissue

Weanling female rats were fed a vitamin D-deficient diet for 4 months until they reached maturity. They were mated with normal, vitamin D-replete male rats and, at 20 days of pregnancy, the female rats were killed and their placentae were removed, homogenzied, and incubated with 25-hydroxyvitamin D3. The incubation mixtures were extracted and the extracts were subjected to Sephadex LH-20 column chromatography followed by high-pressure liquid chromatography. The 1,25-dihydroxyvitamin D3 region of the high-pressure liquid chromatogram was recycled to purity and the structure of the product was identified as 1,25-dihydroxyvitamin D3 by ultraviolet absorption spectrophotometry and by mass spectrometry. Thus it is now evident that placenta, in addition to renal tissue, is capable of converting 25-hydroxyvitamin D3 to the hormonal form of vitamin D, 1,25-dihydroxyvitamin D3.

Acute effects of parathyroid extract on renal vitamin D hydroxylases in Japanese quail

The dose- and the time-response relationships of parathyroid hormone's modulation of in vitro 1,25-(OH)2D3 and 24,25-(OH)2D3 production in Japanese quail were investigated. 4-week-old female Japanese quail were injected intramuscularly with three different doses (30, 90 and 270 USP units/kg) of parathyroid extract (PTH). 4, 12 and 24 h after the in vivo administration of PTH, kidney homogenates were incubated with tritiated 25-(OH)D3. All three doses of PTH stimulated 1,25-(OH)2D3 and inhibited 24,25-(OH)2D3 production at 12 h relative to the vehicle-treated control group. The responses of the vitamin D hydroxylases were not detectable at 4 h and had returned to control levels by 24 h. Plasma calcium rose significantly at 4 h with the two higher doese of PTH (90 and 270 USP units/mg) but not with the lowest dose. By 12 h the plasma calcium had returned to control levels except at the highest dose of PTH. By 24 h all of the plasma calcium levels had returned to control values. Plasma inorganic phosphate levels were depressed at 4 h by all three PTH doses; they remained depressed at 12 h and returned to control levels by 24 h. It is concluded that the pharmacological effects of PTH on the renal vitamin D hydroxylases are slower in onset and shorter in duration when compared with those of estradiol.

Isolation and purification of in vivo-generated 25-hydroxyvitamin D2 by preparative high-performance liquid chromatography

In order to obtain a standard compound of 25-hydroxyvitamin D2 (25-OH-D2), a method for isolating in vivo-generated 25-OH-D2 from the blood of rats or rabbits was established by using several steps of preparative high-performance liquid chromatography (HPLC). When the unsaponifiable matter of the plasma obtained from rats or rabbits receiving a large dose of vitamin D2 was applied to the preparative HPLC using a Zorbax SIL column, a peak denoted as peak X was observed on the chromatogram. Since the peak X was thought to be due to 25-OH-D2 from the experiments of time course and dose-response, it was purified by subjecting it to successive preparative HPLC using several kinds of columns. From the results of ultraviolet (UV) absorption spectrum, gas chromatography-mass spectrometry (GC-MS) and mass chromatography, the purified peak X compound was confirmed to be 25-OH-D2. The proposed method for isolating in vivo-generated 25-OH-D2 is very convenient, because the time to perform each HPLC is very short though several steps of HPLC are used.

Synthesis of 1,25-dihydroxyvitamin D in the nephrectomized pregnant rat

Pregnant rats were maintained on diets either adequate or deficient in vitamin D. On the 20th day of gestation, animals were either nephrectomized bilaterally or sham operated. Immediately therafter, four groups of nephrectomized or sham-operated pregnant rats received iv [26,27-3H]25-hydroxyvitamin D3 ([26,27-3H]25OHD3), while two groups received [1,2-3H,4-14C]D3. The animals were sacrificed 10-24 h later. The distribution of the radiolabeled metabolites of vitamin D3 was determined in extracts of maternal plasma, maternal intestinal tract, placentae, and fetuses after Sephadex LH-20 column chromatography. Both vitamin D3 and 25OHD3 crossed the placenta and entered the fetus. In anephric animals receiving [26,27-3H]-25OHD3, 24,25-dihydroxyvitamin D and a polar peak eluting in the position of 1,25-dihydroxyvitamin D [1,25(OH)2D] and 25,26-dihydroxyvitamin D were identified in extracts of maternal plasma and intestinal tracts and of placentae and fetuses. The identities of 24,25-dihydroxyvitamin D and 1,25 (OH)2D were confirmed by high pressure liquid chromatography. In rats receiving [1,2-3H,4-14C]D3, approximately 50% of the polar metabolite consisted of 1,25(OH)2D. We conclude that the anephric pregnant rat is able to synthesize 1,25(OH)2D, that the fetal portion of the feto-placental unit is the most likely site of production of this hormone, and that this metabolite of vitamin D is able to cross the placenta from the fetus to the mother.

[Parathyroid hormone and 25-hydroxycholecalciferol concentrations in the serum of patients with kidney transplantations and their radiological correlation]

In 40 renal transplant patients the serum concentration of parathyroid hormone, 25-hydroxycholecalciferol, calcium and inorganic phosphate were estimated. The results of these biochemical studies were compared to X-ray soft tissue and bone pathologic findings. The serum immunoreactive parathyroid hormone (iPTH) was normal in 37 patients and increased in 3 cases. 19 (47.5%) renal transplant recipients showed decreased serum 25-hydroxycholecalciferol levels. In 23 out of 40 patients (57.5%) serum phosphate was lower than 3.0 mg/100 ml. X-ray soft tissue changes were observed in 28 (70%) and bone changes in 18 (45%) patients; both frequencies were higher when compared to occurrence during regular dialysis treatment. Features of hyperparathyroid osteopathy were frequently observed in renal transplant recipients. Femoral head necroses were predominant in men, whereas osteomalacia with Looser's zones were found mainly in women. The pathogenesis of X-ray bone changes in patients with renal transplants seems to be multifactorial and related to the duration of secondary hyperparathyroidism before renal transplantation as well as to phosphate depletion and 25-hydroxycholecalciferol deficiency.

The regulation of 24,25-dihydroxyvitamin D3 production in cultures of monkey kidney cells

Primary kidney cell cultures from normal rhesus monkeys were used to study the regulation of 24,25-dihydroxyvitamin D3 production. Kidney cells were grown to confluency in the modified National Cancer Institute Medium NCTC 135 with 2% fetal calf serum and then maintained in a serum-free medium (2% bovine serum albumin) for five additional days prior to a study of regulation. Morphologically, 80% of the cultured cells were epithelial in nautre. These cells produced 24,25-dihydroxy-[3H]vitamin D3 from 25-hydroxy-[3H]-vitamin D3. The identity of the 24,25-dihydroxy-[3H]vitamin D3 was established by Sephadex LH-20 chromatography, by co-chromatography with authentic 24,25-dihydroxyvitamin D3 on high pressure liquid columns, and by periodate sensitivity. Bovine parathyroid hormone at a level of 3 U/ml or human 1-34 parathyroid hormone at 0.05 U/ml for five days suppressed 24,25-dihydroxyvitamin D3 production. Nonradioactive 1,25-dihydroxyvitamin D3 (13 pmol/ml) added once every two days for eight days led to a 2-fold increase in production of 24,25-dihydroxyvitamin D3. Exposure of renal cells to 3 mM instead of 1 mM calcium led to a marked increase in 24,25-dihydroxyvitamin D3 production. These results suggest that renal cell cultures may be an important new approach to a study of regulation of the vitamin D renal hydroxylases.

Maleic acid-induced impaired conversion of 25(OH)D3 to 1,25(OH)2D3: implications for Fanconi's syndrome

Conversion of 25-hydroxyvitamin D3 [25(OH)D3] to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] occurs exclusively in the renal cortex. To determine whether a disorder of the renal cortical tubule capable of causing Fanconi's syndrome can also impair the synthesis of 1,25(OH)2D3, we investigated whether conversion of 25(OH)D3 to 1,25(OH)2D3 was reduced by maleic acid. In vitamin D-deficient rats, maleic acid was administered i.v. over two hours. Thirty minutes after its initiation, when the complex renal tubule dysfunction had occurred, 3H-25(OH)D3 was administered i.v. as a bolus. Five hours afterwards, the amount of 3H-1,25(OH)2D3 recovered in the kidney, small intestine mucosa, and blood was one-third to one-half that in tissues of control rats that received acetazolamide or only saline or were subjected only to the surgical procedure. The glomerular filtration rate, as measured by inulin clearance, did not decrease significantly with maleic acid. In intact vitamin D-deficient chicks, 24 and 22 hr after i.p. administration of maleic acid and 14C-vitamin D3, respectively, the amount of 14C-1,25(OH)2D3 recovered in small intestine mucosa was reduced by one-half when compared to saline-treated controls. In kidney homogenates and isolated renal tubules of vitamin D-deficient chicks, activity of 25-hydroxyvitamin D3-1-hydroxylase was diminished immediately after maleic acid was administered in vivo or added in vitro to the incubation medium, respectively. These data provide the first demonstration that the renal capacity to convert 25(OH)D3 to 1,25(OH)2D3 can be substantially impaired in vivo by a renal disorder in which the glomerular filtration rate is not reduced.

In vivo evidence for the intermediary role of 3',5'-cyclic AMP in parathyroid hormone-induced stimulation of 1alpha,25-dihydroxyvitamin D3 synthesis in rats

Teh stimulatory effect of parathyroid hormone (PTH) on renal 1alpha-hydroxylation of 25-hydroxyvitamin D3 (25-OH-D3) was studied in thyro-parathyroidectomized (TPTX), vitamin D-deficient rats into which bovine PTH, theophylline, cAMP or dibutyryl cAMP (dbcAMP) was constantly infused. The accumulation in plasma of 1alpha,25-dihydroxy-vitamin D3 [1alpha,25-(OH)2-D3], produced from 25-OH-D3, was enhanced by infusion of either cAMP (0.9 MUMOL/H) OR DBCAMP (1 mumol/h) to a level similar to the maximum obtained by PTH (i95--7.5 U/h) infusion. A submaximal dose (1 U/h) of PTH caused a similar extent of stimulation, when infused with theophylline. When either 2 mumol/h of cAMP or 7.5 U/h of bovine PTH was infused starting 18 h after TPTX, the accumulation of 1alpha,25-(OH)2-D3 in plasma was similarly restored withing 6 h to the level found in the sham-operated animals. These results strongly support the concept that cAMP plays an important intermediary role in the stimulation of 1alpha,25-(OH)2-D3 production induced by both exogenous and endogenous PTH.