Specific binding of 1alpha,25-dihydroxycholecalciferol to nuclear components of chick intestine.

Specific binding of 1alpha,25-dihydroxycholecalciferol to macromolecular components of small intestinal mucosa nuclei is demonstrated in vitamin D-deficient chicks. The nuclear 1alpha,25-dihydroxycholecalciferol-macromolecule complex was isolated on sucrose density gradients and sediments at 3.7 S in the presence of 0.3 M KCl. Agarose gel filtration of the nuclear component indicated an apparent molecular weight of 47,000. The nuclear receptor complexes could not be distinguished from previously described cytoplasmic 1alpha,25-dihydroxycholecalciferol-binding components by the ultracentrifugation and chromatographic procedures employed. The association of the 3-H-sterol with the nuclear component is thermolabile and is destroyed by treatment with pronase, but not by nucleases; the receptor component is therefore presumed to be a protein. The macromolecular-1alpha,25-dihydroxycholecalciferol complex formed in vivo or in vitro at 25 degrees can be extracted from intestinal nuclei by 0.3 M KCl, but not by low salt buffers. Smaller amounts of the 3.7 S binding component can be detected in isolated purified chromatin or after incubation of 1alpha,25-dihydroxy[3-H]cholecalciferol with reconstituted cytosol-chromatin at 0 degrees. Following incubation of the labeled hormone with reconstituted cytosol-chromatin at 0 degrees, 1alpha,25-dihydroxy[3-H]cholecalciferol is primarily associated with the cytoplasmic receptor, After shifting the incubation temperature to 25 degrees, a progressive increase in the concentration of the nuclear receptor complex and a concomitant decrease in the concentration of the cytoplasmic binding component occur. Thus the 1alpha,25-dihydroxycholecalciferol binding molecules appear to exist primarily in the cytoplasm, where they presumably function to transport the hormone into the nucleus. Experiments employing incubation of 1alpha,25-dihydroxy[3-H]cholecalciferol with reconstituted cytosol-chromatin from nontarget tissues indicate a requirement for both intestinal cytosol and chromatin for maximal formation of the nuclear hormone-receptor complex. These results suggest that the nuclear-binding component arises from hormone-dependent transfer of the cytoplasmic 1alpha,25-dihydroxycholecalciferol receptor to intestinal chromatin acceptor sites.     

Synthesis of 1 alpha-hydroxy[7-3H]cholecalciferol and its metabolism in the chick.

1. 1 alpha-Hydroxy[7-3H]cholecalciferol (specific radioactivity of 2-Ci/mmol) was synthesized, and its metabolism in chicks studied. 2. 1 alpha-Hydroxy[7-3H]cholecalciferol was metabolized very rapidly in the chick to 1 alpha,25-dihydroxy[7-3H]cholecalciferol and to a metabolite less polar than 1 alpha-hydroxycholecalciferol. Intestine exhibited highest accumulation of 1 alpha-25-dihydroxy[7-3H]cholecalciferol, and liver exhibited highest accumulation of the non-polar metabolite. 3. Tissue uptake of 1 alpha-hydroxy[7-3H]cholecalciferol and its metabolites in chicks that were dosed continuously for 16 days with 1 alpha-hydroxy[7-3H]cholecalciferol did not exceed by very much that observed in tissues obtained from chicks that were dosed with a single injection of 1 alpha-hydroxy[7-3H]cholecalciferol 24 h before killing, except for liver and kidney. 4. Lowest accumulation of metabolites was noted in muscle and bone, and for the latter, highest uptake of 1 alpha,25-dihydroxy[7-3H]cholecalciferol was noted in the epiphysial periosteum and the metaphysis. 5. Formation of 1 alpha,24,25-trihydroxy[7-3H]cholecalciferol was not observed in the chicks that were dosed continuously with 1 alpha-hydroxy[7-3H]cholecalciferol, despite the fact that plasma calcium and phosphorus were normal and despite the presence of renal 24-hydroxylase activity. 6. The vitamin D status of the chicks did not appear to affect the metabolic profile of the administered 1 alpha-hydroxy[7-3H]cholecalciferol.     

Production of C-24- and C-23-oxidized metabolites of 1,25-dihydroxycholecalciferol by cultured kidney cells (LLC PK1) and their presence in kidney in vivo.

1,25-Dihydroxy[3H]cholecalciferol was converted into several more-polar metabolites by a cultured pig kidney cell line (LLC PK1). The production of metabolites was stimulated by pretreating the cells with unlabelled 1,25-dihydroxycholecalciferol. A similar profile of metabolites was observed on high-pressure-liquid-chromatographic analysis of an extract from the kidneys of rats dosed intravenously with 1,25-dihydroxy[3H]cholecalciferol. Among the metabolites detected were 1,24,25-trihydroxycholecalciferol, 1,25-dihydroxy-24-oxocholecalciferol, 1,23,25-trihydroxy-24-oxocholecalciferol and 1,25-dihydroxycholecalciferol-26,23-lactone. The results are in accord with data reported for intestinal 1,25-dihydroxycholecalciferol metabolism [Napoli, Pramanik, Royal, Reinhardt & Horst (1983) J. Biol. Chem. 258, 9100-9107]. These data indicate that C-23- and C-24-oxidation of 1,25-dihydroxycholecalciferol are phenomena common to calciferol target tissues, and that regulation of 1,25-dihydroxycholecalciferol homoeostasis is dependent on the rate of its metabolism in addition to the rate of its synthesis.     

Cholecalciferol metabolites binding in porcine parathyroid glands.

We studied the cytoplasmic and nuclear binding of 25-hydroxychole-calciferol and 1alpha,25-dihydroxycholecalciferol inside porcine parathyroid glands. Both sterols bind to cytoplasmic components, but a specific nuclear uptake was demonstrated only for 1alpha,25-dihydroxycholecalciferol. These findings support the hypothesis that mammalian parathyroid glands are a target organ for some cholecalciferol metabolites.     

1 alpha,25-Dihydroxycholecalciferol and a human myeloid leukaemia cell line (HL-60).

Human promyelocytic leukaemia cells (HL-60) can be induced to differentiate into mature granulocytes in vitro by 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3], the active form of cholecalciferol. The differentiation-associated properties, such as phagocytosis and C3 rosette formation, were induced by as little as 0.12 nM-1 alpha,25(OH)2D3, and, at 12 nM, about half of the cells exhibited differentiation on day 3 of incubation. Concomitantly the viable cell number was decreased to less than half of the control. Among various derivatives of cholecalciferol examined, 1 alpha,25(OH)2D3 and 1 alpha,24R-dihydroxycholecalciferol were the most potent in inducing differentiation, followed successively by 1 alpha,24S-dihydroxycholecalciferol, 1 alpha-hydroxycholecalciferol, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol. A cytosol protein specifically bound to 1 alpha,25 (OH)2D3 was found in HL-60 cells. Its physical properties closely resembled those found in such target tissues as intestine and parathyroid glands. 1 alpha,25(OH)2D3 bound to the cytosol receptor was transferred quantitatively to the chromatin fraction. The specificity of various derivatives of cholecalciferol in inducing differentiation was well correlated with that of their association with the cytosol receptor. These results are compatible with the hypothesis that the active form of cholecalciferol induces differentiation of human myeloid leukaemia cells by a mechanism similar to that proposed for the classical concept of steroid hormone action.     

The localization of 1,25-dihydroxycholecalciferol in bone cell nuclei of rachitic chicks

1. A simple technique has been developed to obtain subcellular fractions of chick bone. The method yielded 60-70% of total DNA in the nuclear debris fraction and 80-90% of total (14)C recovered in bone after a dose of radioactive vitamin D. 2. After a dose of [4-(14)C,1,2-(3)H(2)]cholecalciferol (0.5mug) was given to vitamin D-deficient chicks, the time-course of total (14)C radioactivity in the epiphysis, metaphysis and diaphysis of proximal tibiae was measured. The maximum concentrations were reached at 6h, corresponding to a similar peak of radioactivity in blood, decreasing until 24h and indicating the dependence on the circulating (14)C and on the blood supply of the three bone components. 3. The (14)C radioactivity of cholecalciferol and 25-hydroxycholecalciferol (expressed per mg of DNA) followed the pattern of incorporation of total (14)C radioactivity in all three bone components. The more polar metabolite fraction reached a peak of radioactivity at 6-9h and maintained its concentration over the 24h period studied in all anatomical bone components. 4. After a dose of [4-(14)C,1-(3)H]cholecalciferol (0.5mug) was given to vitamin D-deficient chicks, the subcellular distribution was studied. At 24h after dosing, the nuclear fraction contained 27% and the supernatant fraction had 67% of total (14)C recovered in the bone filtrate. When the (14)C in the residual bone fragments was included, the nuclear fraction contained up to 35% of the total radioactivity in the bone. 5. The subcellular distribution pattern of individual vitamin D metabolites indicated that the purified nuclear fraction concentrated the polar metabolite, which lost (3)H at C-1, so that 77% of the radioactivity could be accounted for by 1,25-dihydroxycholecalciferol. The supernatant fraction contained smaller amounts of 1,25-dihydroxycholecalciferol (9%), with 66% of 25-hydroxycholecalciferol forming the major metabolite, corresponding to its concentration found in blood at 24h. 6. The preferential accumulation of 1,25-dihydroxycholecalciferol in the nuclear fraction and the overall pattern of other metabolites, found previously in intestinal tissue, suggests a similar mechanism of action in bone to that postulated for the intestinal cell in calcium translocation.     

The effects of 24R,25-dihydroxycholecalciferol and of 1 alpha,25-dihydroxycholecalciferol on ornithine decarboxylase activity and on DNA synthesis in the epiphysis and diaphysis of rat bone and in the duodenum.

The effect of cholecalciferol metabolites on ornithine decarboxylase activity and on DNA synthesis in developing long bones was investigated in vitamin D-depleted rats. In the epiphysis there was a 6.4-fold increase in ornithine decarboxylase activity 5 h after a single injection of 24R,25-dihydroxycholecalciferol but not of 24S,25-dihydroxycholecalciferol or other vitamin D metabolites. In comparison, in the diaphysis and duodenum, 1 alpha,25-dihydroxycholecalciferol, but not other vitamin D metabolites, caused a 3-3.5-fold increase in the enzyme activity. The enzyme activity in the tissues examined attained a maximal value at 5 h after the injection of the metabolites. The activity of ornithine decarboxylase in the epiphysial region increased dose-dependently as the result of a single injection of 24R,25-dihydroxycholecalciferol and attained a maximal value at a dose between 30 and 3000 ng. In addition, administration of 24R,25-dihydroxycholecalciferol, but not 24S,25-dihydroxycholecalciferol or other metabolites, caused within 24 h a 1.7-2.0-fold increase in [3H]thymidine incorporation into DNA of the epiphyses of tibial bones. In comparison, 1 alpha,25-dihydroxycholecalciferol caused a 1.5-fold increase in [3H]thymidine incorporation into DNA of the diaphyses and of the duodenum. The present data indicate that 24R,25-dihydroxycholecalciferol is involved in the regulation of epiphyseal growth, whereas 1 alpha,25,dihydroxycholecalciferol stimulates the proliferation of cells in the diaphysis of long bones and in the intestinal mucosa.     

Comparative studies on the 25-hydroxylations of cholecalciferol and 1α-hydroxycholecalciferol in perfused rat liver

The 25-hydroxylations of [(3)H]cholecalciferol and 1alpha-hydroxy[(3)H]cholecalciferol in perfused rat liver were compared. Results showed that about twice as much 1alpha(OH)D(3) (1alpha-hydroxycholecalciferol) was incorporated into the liver as cholecalciferol. 25-Hydroxy[(3)H]cholecalciferol and 1alpha-25-dihydroxy[(3)H]cholecalciferol were not incorporated significantly. Livers isolated from vitamin D-deficient rats formed the 25-hydroxy derivatives of cholecalciferol and 1alpha(OH)D(3) respectively linearly with time for at least 120min. The rate of 1alpha,25(OH)(2)D(3) (1alpha,25-dihydroxycholecalciferol) production increased exactly 10-fold on successive 10-fold increases in the dose of 1alpha(OH)D(3), suggesting that hepatic 25-hydroxylation of 1alpha(OH)D(3) is not under metabolic control. On the other hand, the rate of conversion of cholecalciferol into 25(OH)D(3) (25-hydroxycholecalciferol) did not increase linearly with increase in the amount of cholecalciferol in the perfusate. The 25-hydroxylation of cholecalciferol seemed to proceed at a similar rate to that of 1alpha(OH)D(3) at doses of less than 1nmol, but with doses of more than 2.5nmol, the conversion of cholecalciferol into 25(OH)D(3) became much less efficient, though the linear relation between the amounts of substrate and product was maintained. A reciprocal plot of data on the 25-hydroxylation of cholecalciferol gave two K(m) values of about 5.6nm and 1.0mum, whereas that for the 25-hydroxylation of 1alpha(OH)D(3) gave a single K(m) value of about 2.0mum. These results suggest that there are two modes of 25-hydroxylation of cholecalciferol in the liver, which seem to be closely related to the mechanism of control of 25(OH)D(3) production by the liver.     

Vitamin D metabolism and its possible role in the developing chick embryo.

The relationship between bone formation and vitamin D metabolism was investigated in the developing chick embryo. Fertilized White Leghorn eggs were incubated at 38 degrees C in an incubator for 21 days. The fresh weight and calcium content of embryonic tibiae began to increase at day 12 and attained maximal values at day 19. Bone alkaline phosphatase and citrate decarboxylation activities, both of which represent osteoblastic activity, also began to increase at days 10-12, reached maximal values at day 19 and sharply declined thereafter. Both bone enzyme activities were highly correlated with CA2+-binding activity in the chorioallantoic membrane measured by the Chelex 100 assay. When mesonephric and metanephric homogenates were incubated with 25-hydroxy[3H]cholecalciferol, a marked and concomitant increase occurred in the metanephric 1 alpha- and 24-hydroxylase activity after day 14. The production of 1 alpha, 25-dihydroxycholecalciferol attained a maximal value at day 19 and decreased thereafter, whereas that of 24,25-dihydroxycholecalciferol continued to increase until hatching. The production rate of 1 alpha, 25-dihydroxycholecalciferol by the metanephros coincided with the changes in Ca2+-binding activity in the chorioallantoic membrane and osteoblastic activity. Since both intestinal calcium absorption and bone mineral mobilization do not occur in embryonic life, these results support the idea that 1 alpha, 25-dihydroxycholecalciferol may be involved directly in bone formation or induction of a calcium-binding protein in the chorioallantoic membrane.     

Pituitary 1,25-dihydroxyvitamin D3 receptors in hyperthyroid- and hypothyroid-rats

The binding of 1 alpha,25-dihydroxy (26,27-methyl-[3H]) cholecalciferol ([3H]1,25-(OH)2D3) to its receptor in cytosol of the anterior pituitary cells was examined in hyperthyroid- and hypothyroid rats, as well as in normal rats. The binding capacity increased by 41% in L-Thyroxine-treated hyperthyroid rats and decreased by 49% in propylthiouracil-ingested hypothyroid rats as compared with normal control rats, whereas the affinity of the receptor for [3H]-1,25(OH)2D3 showed no difference among these 3 animal groups. These findings indicate that the number of 1,25(OH)2D3 receptors in the pituitary may be regulated by thyroid hormone, and further suggest that 1,25-(OH)2D3 may play some role in regulating functions of the anterior pituitary.     

Metabolism of 25-hydroxycholecalciferol in human promyelocytic leukemia cells (HL-60). Isolation and identification of (5Z)- and (5E)-19-nor-10-oxo-25-hydroxycholecalciferol

Human promyelocytic leukemia cells incubated with 25-hydroxy[26,27-methyl-3H] cholecalciferol (1 microCi) or non-radioactive 25-hydroxycholecalciferol (550 micrograms) produced significant quantities of two vitamin D3 metabolites. The two metabolites were isolated and purified by methanol chloroform extraction and a series of chromatographic procedures. The metabolite purification and elution positions on these columns were followed by radioactivity and their ultraviolet absorption at 310 nm. The two metabolites have been unequivocally identified as (5Z)- and (5E)-19-nor-10-oxo-25-hydroxycholecalciferol by ultraviolet absorption spectrophotometry, mass spectrometry, Fourier-transform infrared spectrophotometry and co-chromatography with synthetic compounds on a high-performance liquid chromatograph. (5E)- but not (5Z)-19-nor-10-oxo-25-hydroxycholecalciferol was able to induce HL-60 cell phenotypic and functional differentiation. However, these two metabolites of 25-hydroxycholecalciferol did not bind specifically to the chick intestinal 3.7 S. receptor protein for 1 alpha,25-dihydroxycholecalciferol. The precise biological role of these metabolites is as yet unclear.     

The metabolism of cholecalciferol in the liver of Japanese quail (Coturnix coturnix japonica) with particular reference to the effects of oestrogen.

1. Studies were carried out in vitro with the livers of Japanese quail that had been fed from hatching on diets supplying their full requirements for vitamin D. 2. 25-Hydroxycholecalciferol was the major metabolite when liver homogenates of egg-laying female and oestrogen-treated quail of both sexes were incubated with [3H]cholecalciferol. 3. Very little 25-hydroxycholecalciferol was generated from liver homogenates of adult male and immature quail. Instead the cholecalciferol was converted into one or more compounds less polar than 25-hydroxycholecalciferol and into a number of highly polar metabolites, some of which were water-soluble. 4. Oestrogen not only stimulated the 25-hydroxylation of cholecalciferol but also protected both cholecalciferol and 25-hydroxycholecalciferol from degradation by the enzymic pathways active in immature and male birds. 5. These actions of oestrogen may be of physiological significance in relation to the high requirements of laying birds for 1,25-dihydroxycholecalciferol to support the intense metabolism of calcium associated with egg-shell calcification.     

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.     

Intranuclear localization and receptor proteins for 1,25-dihydroxycholecalciferol in chick intestine

1. The intranuclear distribution of cholecalciferol and its metabolites was studied in the intestine of rachitic chicks. 2. At high doses of cholecalciferol the nuclei contain the vitamin and its 25-hydroxy metabolite, but over 80% of this is localized on the nuclear membranes. The hormone, 1,25-dihydroxycholecalciferol, is found within the cell nuclei irrespective of the intake of cholecalciferol, but significant amounts could not be found with chromatin isolated free of nuclear membranes. 3. 1,25-Dihydroxycholecalciferol is associated in the nucleus with an acidic protein. Since one of the actions of 1,25-dihydroxycholecalciferol is to control the synthesis of mRNA for calcium-binding protein it was to be expected that the hormone would be bound to chromatin, as with the other steroid hormones. It is suggested that the hormone-receptor complex exists as part of an equilibrium mixture of the complex bound to the DNA and in a free form. 4. A protein extract of nuclei was obtained, which when incubated at 4 degrees C for 1h took up the 1,25-dihydroxycholecalciferol. The nature of this binding was studied. 5. There appear to be two nuclear proteins able to bind the hormone one of which is the intestinal nuclear receptor. The binding sites on this protein are saturable with the hormone, have an association constant of 2x10(9)m(-1) and show a high chemical specificity for the 1,25-dihydroxycholecalciferol. The number of nuclear binding sites for the hormone provided by this receptor is similar to the maximum intestinal hormone concentration so far observed. Its sedimentation coefficient is 3.5S, and is very close to that observed for the nuclear protein to which is attached the 1,25-dihydroxycholecalciferol formed in vivo from vitamin D. 6. The cytoplasmic protein has an association constant of 1x10(9)m(-1)and a sedimentation coefficient of 3.0S, but its relation with the nuclear receptor is not yet clear.     

Structural requirements for the interaction of 1 alpha, 25-(OH) 2- vitiamin D3 with its chick interestinal receptor system.

The mechanism by which 1 alpha,25-dihydroxycholecalciferol (1alpha,25-(OH)2D3), the biologically active metabolite of cholecalciferol (vitamin D3), stimulates intestinal calcium absorption has been shown to involve an interaction of the steroid with a specific cytosol-chromatin receptor system in this target organ. Thus, 1alpha,12(OH)2D3 binds to a specific cytoplasmic receptor protein and then, following a temperature-dependent step, becomes associated with a finite number of chromatin acceptor sites prior to the initiation of the physiological response. In this respect, 1alpha,25(OH) 2D3 is similar to a number of other steroid hormones. In this investigation, studies were performed to help define the essential structural features required for interaction of 1alpha,25-(OH)2D3 with its intestinal receptor system, and presumably, for biological activity. To this end, competition studies utilizing a series of closely related structural analogs of cholecalciferol were carried out by means of a competitive binding assay highly specific for 1alpha,25(OH)2D3. The competitive binding assay employed in this study is dependent upon the ability to duplicate, in vitro, the conditions which permit the saturable binding of 1alpha,25-(OH)2[3H]D3 to chick intestinal chromatin, in vivo. Optimal conditions for this assay were achieved by the incubation of a reconstituted intestinal receptor system consisting of separately isolated cytosol and Triton X-100 chromatin fractions at 25 degrees for 45 min with 2.0 X 10-8 M 1alpha,25-(OH)2[3H]D3. Maximal binding of about 21 to 24 pmol of radioactive steroid bound per chick intestinal chromatin occurred under these conditions. The ability of the various analogs to compete with the radioactive hormone was assessed by virtue of a decrease in the amount of radioactive steroid bound to the chromatin in the presence of increasing concentrations of nonradioactive analog.     

The mechanism of end-organ resistance to 1 alpha,25-dihydroxycholecalciferol in the common marmoset.

The common marmoset, a New World monkey, requires a large amount of cholecalciferol (110 i.u./day per 100g body wt.) to maintain its normal growth. In a previous report, we demonstrated that the circulating levels of 1 alpha, 25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] in the marmosets are much higher than those in rhesus monkeys and humans, but the marmosets are not hypercalcaemic [Shinki, Shiina, Takahashi, Tanioka, Koizumi & Suda (1983) Biochem. Biophys. Res. Commun. 14, 452-457]. To compare the effect of the daily intake of cholecalciferol, two rhesus monkeys were given a large amount of cholecalciferol (900 i.u./day per 100g body wt). Their serum levels of calcium, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol were markedly elevated, but the serum 1 alpha,25(OH)2D3 levels remained within a range similar to those in the rhesus monkeys fed the normal diet (intake of cholecalciferol 5 i.u./day per 100g body wt). Intestinal cytosols prepared from both monkeys contained similar 3.5 S macromolecules to which 1 alpha,25(OH)2D3 was bound specifically. However, the cytosols from the marmosets contained only one-sixth as many 1 alpha,25(OH)2D3 receptors as those from the rhesus monkeys. Furthermore, the activity of the 1 alpha,25(OH)2D3-receptor complex in binding to DNA-cellulose was very low in the marmosets. These results suggest that the marmoset possesses an end-organ resistance to 1 alpha,25(OH)2D3 and is a useful animal model for studying the mechanism of vitamin D-dependent rickets, type II.     

Estrogen 1,2-epoxides or estrogen quinones/semiquinones

Metabolic activation of estradiol leading to the formation of catechol estrogens is a prerequisite for its genotoxic activity. Both estrogen-o-quinones/semiquinones and estrogen 1,2-epoxides have been proposed to be responsible for this activity. Incubations of [3H]estradiol and [3H]1 alpha,2 alpha-epoxy-4-estrene-3-one-17 beta-ol (ketotautomer of estradiol 1,2-epoxide) with rat liver microsomal and cytosol preparations were carried out in the presence of SKF 525A, ascorbic acid, glutathione and cysteine. Ascorbic acid decreased binding to proteins and aqueous-soluble fraction with both [3H] estradiol and [3H]epoxyestrenolone in incubations with microsomes but no effect with cytosol fraction. Incubations of microsomes with thiols gave water-soluble metabolites which were characterized as 1(4)-thioether derivatives of 2-hydroxyestradiol and incubations of [3H]epoxyestrenolone with cytosol and thiols gave estradiol-2-thioether. Incubations with ascorbic acid and thiols resulted in decreased formation of water-soluble metabolites in microsomal incubations but not in cytosol incubations. These studies indicate that the major pathway for irreversible binding of estrogens to macromolecules involves estrogen-o-quinones/semiquinones and not estrogen 1, 2-epoxide.     

Substrate-binding ability of Escherichia coli ribonucleic acid polymerase in relation to its protein composition.

The metabolism of [3H]progesterone in the rabbit endometrium and myometrium was studied in vitro. The major metabolities identified were 5alpha-pregnane-3,20-dione, 20alpha-hydroxypregn-4-en-3-one, 3beta-hydroxy-5alpha-preganan-20-one and 5alpha-pregnane-3beta,20alpha-diol. Other minor metabolites tentatively identified were 3alpha-hydroxy-5beta-pregnan-20-one,20alpha-hydroxy-5beta-pregnan-3-one and 5beta-pregnane-3alpha,20alpha-diol. The ability of the endometrium to metabolize progesterone on a unit weight bais was about 2.7 times that of the myometrium. The metabolism of [3H]progesterone in the rabbit uterus under the influnce of oestradiol-17beta and progesterone was studied. The ability of the oestradiol-treated rabbit uterus to metabolize progesterone was increased to 3.47 times that of the overiectomized control uterus, whereas the oestradiol-progesterone-treated rabbit uterus metabolized only 1.86 times that of the control. Study of the metabolism of progesterone with uterine subcellular preparations revealed that the 5alpha-reductase enzyme was present mainly in the nuclear fraction; 20alpha-hydroxysteroid dehydrogenase was found in the cytosol fraction and 3beta-hydroxysteroid dehydrogenase in the particulate fraction of the uterus. The metabolic pathways of progesterone in the rabbit uterine tissue are discussed.     

Metabolism of 1alpha-hydroxyvitamin D3 in the chick.

Chicks convert both orally and intravenously administered 1alpha-hydroxy[6-3H]vitamin D3 rapidly to 1alpha,25-dihydroxy[6-3H]vitamin D3. The maximal accumulation of 1alpha,25-dihydroxy[6-3H]vitamin D3 in intestine precedes the intestinal absorption response to 1alpha-hydroxyvitamin D3 by at least 2 hours. Oral administration results in the highest concentrations of 1alpha,25-dihydroxy[6-3H]vitamin D3 in intestine, giving a level about 1.5 times that achieved with an intravenous dose. On the other hand, an oral dose of 1alpha-hydroxy[6-3H]vitaminD3 gives much lower amounts of both 1alpha-hydroxy[6-3H]vitamin D3 and 1alpha,25-dihydroxy[6-3H]vitamin D3 in bone and blood than an intravenous dose, which suggests that the 1alpha-hydroxy[6-3H]vitamin D3 may not be utilized as well by the oral route as by an intravenous route. Liver homogenates from both rat and chick convert 1alpha-hydroxy[6-3H]vitamin D3 to 1alpha,25-dihydroxy[6-3H]vitamin D3. However, intestinal homogenates from chick, but not rat, can also cary out this conversion, which may account for the higher concentration of 1alpha,25-dihydroxy[6-3H]vitamin D3 found in the intestine of chicks given an oral dose of 1alpha-hydroxy[6-3H]vitamin D3.     

Investigations on metabolites of vitamin D in rat bile. Separation and partial identification of a major metabolite

1. Young rats with cannulated bile ducts were given 0.34mg. of [1alpha-(3)H]cholecalciferol or 0.54mg. of [(14)C]ergocalciferol by intravenous infusion. Of the radioactivity in the dose of [1alpha-(3)H]cholecalciferol 31% was recovered in bile within 24hr. 2. The metabolites in bile were separated by gradient-elution column chromatography on silicic acid into five components, all more polar than cholecalciferol or 25-hydroxycholecalciferol. [(14)C]Ergocalciferol gave a similar pattern of metabolites in bile. 3. The three most polar metabolites were shown to be ionic. The major component has been identified as a glucuronide conjugate, which was not identical with synthetic cholecalciferyl glucuronide.