10-Keto or 25-hydroxy substitution confer equivalent in vitro bone-resorbing activity to vitamin D3

Characterization of 19-nor-10-oxo-25-hydroxyvitamin D3 production by solubilized chick kidney mitochondria and bovine serum albumin

The vitamin D3 metabolite obtained from the incubation of 3-[(cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO)-solubilized chick kidney mitochondria with 25-hydroxyvitamin D3 (25-OH-D3) was identified to be 5(E)-19-nor-10-oxo-25-hydroxyvitamin D3 (5(E)-19-nor). The production of 19-nor was dependent on time and on protein concentration, but was not dependent on the pH of the incubation. 19-Nor was not formed in the absence of protein or when protein had been heat-treated following detergent solubilization. 19-Nor was not further metabolized to any other product upon incubation with the CHAPSO-solubilized proteins. No 19-nor-10-oxo derivative of 1,25(OH)2D3 was formed when 1,25(OH)2D3 was used as substrate in the incubation. Kinetic analysis showed a substrate saturation with an apparent Vmax of about 4.1 pmol/min.mg and S0.5 of approximately 1.3 x 10(-6) M. The production of 19-nor was not restricted to the CHAPSO-soluble protein fraction of kidney mitochondria but was also found in both the CHAPSO-soluble and -insoluble fractions of chick liver mitochondria and CHAPSO-treated bovine serum albumin (BSA). 19-Nor production by detergent-treated BSA also showed saturation kinetics with a similar S0.5 and an apparent Vmax which was about 5-fold higher than that obtained with CHAPSO-solubilized mitochondria. The evidence suggests that the formation of 19-nor is not mediated by a traditional enzyme, but does require protein. A mechanism for the conversion of 25-OH-E3 to 19-nor is proposed, in which the naturally-occurring 5(Z)-25-OH-D3 substrate binds to protein, isomerizes to 5(E)-25-OH-D3 and is oxidized by hydrogen peroxide to 5(E)-19-nor via a dioxetane intermediate.

The biological assessment of vitamin D3 metabolites produced by rumen bacteria

Biological assays were performed to evaluate 10-oxo-19-nor-vitamin D3 (10-oxo-D3) and 5(E) 25-hydroxy-10-oxo-19-nor-vitamin D3 (25-OH-10-oxo-D3) two bacterial products of vitamin D3 (D3) and 25-hydroxyvitamin D3 (25-OHD3) metabolism, respectively. The 5(Z) and 5(E) isomers of 10-oxo-D3 were, respectively, 40- and 80-fold less active than D3 in stimulating Ca+2 absorption from the gut. 25-Hydroxy-10-oxo-D3 did not stimulate Ca+2 absorption. Only 5(Z) 10-oxo-D3 induced mobilization of bone Ca+2. In addition, both 10-oxo-D3 and 25-OH-10-oxo-D3 showed poor affinities for either the plasma D3-binding protein or the thymus 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor. 10-Keto-D3 exhibited a plasma half-life of only 6 min. This was a much shorter half-life than that exhibited by other vitamin D metabolites and was expected because of the poor affinity 10-oxo-D3 has for the plasma vitamin D binding protein. Bacterial metabolism of D3 deactivates the vitamin, which allows ruminants to tolerate relatively large oral doses of D3.

Metabolism of the vitamin D3 derivative (24R)-hydroxycalcidiol by human promyelocytic leukemia cells (HL-60). Isolation and identification of (5Z) and (5E)-(24R)-19-nor-10-oxo-24-hydroxycalcidiol

Human promyelocytic leukemia cells (HL-60 cells) incubated with (24R)-hydroxy[26,27-methyl-3H]calcidiol (0.2 microCi) or non-radioactive (24R)-hydroxycalcidiol (370 micrograms) produced significant quantities of two new vitamin D3 (calciol) metabolites. The metabolites were isolated from HL-60 cell culture media by methanol/chloroform extraction and a series of chromatographic procedures. The two new metabolites were identified as (5Z)- and (5E)-(24R)-19-nor-10-oxo-24-hydroxycalcidiol by HPLC analysis, ultraviolet absorption spectrophotometry, mass spectrometry and Fourier-transform infrared spectrophotometry. According to the isolation and purification procedures, the total amounts of 3.04 micrograms (5Z)-(24R)-19-nor-10-oxo-24-hydroxycalcidiol (lambda max = 310 nm, epsilon = 17070 M-1 cm-1) and 8.89 micrograms (5E)-(24R)-19-nor-10-oxo-24-hydroxycalcidiol (lambda max = 312 nm, e = 24,500 M-1 cm-1) were calculated, assuming an Mr of 418. The activity of 19-nor-10-oxo-(24R)-hydroxycalcidiol to promote HL-60 cell differentiation was higher than the activity of the precursor (24R)-hydroxycalcidiol suggesting a possible biological action of this metabolite in HL-60 cells.

Biological activity of 19-nor, 10-keto, 25-hydroxyvitamin D3

19 nor, 10 keto, 25-hydroxyvitamin D3 (19/10-25OHD3) is a metabolite of 25-OHD3 produced in vitro by various phagocytes including normal human blood monocytes and transformed cell lines, U937 and HL-60. We recently reported that 19/10-25OHD3, induced differentiation of U937 cells. In these studies, 19/10-25OHD3 alone produced no detectable effect on the growth rates, surface adherence, and oxidative metabolism of U937 and HL-60 cells. When combined with lymphocyte-conditioned medium (LCM), 19/10-25OHD3 reduced proliferation, increased surface adherence and stimulated luminol-dependent luminescence (LDL) of the U937 cells. In contrast, the combination of 19/10-25OHD3 and LCM had no effect on the growth of HL-60 cells but did increase the surface adherence and the expression of a complement receptor component. 19/10-25OHD3 competed for tritium-labeled 1,25(OH)2D3 binding to receptors extracted from cultured human skin fibroblasts. This displacement capacity was 600 times weaker than that of unlabeled 1,25(OH)2D3. Incubation of human skin fibroblasts for 24 hr with 19/10-25OHD3 induced 25OHD3-24-hydroxylase activity in the fibroblasts. The inductive potency of 19/10-25OHD3 was 1/50 that of 1,25(OH)2D3. These results demonstrate bioactivity of 19/10-25OHD3 in several systems. At least one of these responses, the induction of 25OHD3-24-hydroxylase, is a receptor-mediated event. Some of the other responses may be independent of the cellular receptor for 1,25(OH)2D3. Interestingly, the potency of 19/10-25OHD3 was highest in the receptor-mediated response (1:50) and lower in the other parameters, ranging from 1:100 to 1:600 compared to 1,25(OH)2D3. This range of bioactivity in phagocytes and fibroblasts is presently explained.

25-Hydroxyvitamin D3 metabolism in a human leukemia cell line

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is a potent inducer of monocytic differentiation of the human promyelocytic leukemia cell line, HL-60. We have noted that 25-hydroxyvitamin D3 (25(OH)D3) in high doses is also capable of promoting monocytic differentiation of this cell line. To test the possibility that the latter activity is due to conversion of 25OHD3 to 1,25(OH)2D3 by HL-60, we exposed HL-60 cells to 25OHD3 and analyzed the products by HPLC and radioreceptor assay. When chromatographed in the traditional solvent system (isopropanol-hexane), a new peak appears which migrates with authentic 1,25(OH)2D3. However, in a solvent system containing dichloromethane, 90% of the peak migrates with another metabolite, 19-Nor-10-Keto-25OHD3 (19-Nor-25OHD3). Production of this metabolite is enhanced by living cells and is synthesized by both virgin HL-60 and those which have undergone differentiation. We next determined if authentic 19-Nor-25OHD3 also promotes differentiation of this cell. As assessed by appearance of the monocyte-specific surface antigen (63D3) and macrophage-specific esterase activity, we find that this metabolite does, in fact, induce monocytic differentiation of HL-60 with a potency of approximately 1/200 that of 1,25(OH)2D3 and similar to that of 25OHD3. In agreement with the effect upon cell maturation, 19-Nor-25OHD3 displaces 3H-1,25(OH)2D3 from its HL-60 receptor with an efficiency comparable to 25OHD3. Hence, HL-60 cells convert 25OHD3 to 19-Nor-25OHD3, and 19-Nor-25OHD3 induces monocytic differentiation of HL-60 with comparable efficiency to its precursor, 25OHD3.

Phagocytic cells synthesize 19-nor-10-keto-25-hydroxyvitamin D3, a metabolite that may induce differentiation of the human monoblastic cell line U937

Phagocytic cells, including normal human blood neutrophils and monocytes, metabolize 25-hydroxyvitamin D3 in vitro to more polar metabolites. Cells of the human monoblastic cell line U937 produced three metabolites when incubated with 25-hydroxyvitamin D3. One of these metabolites, previously designated peak III, has its maximal absorbance at 310 nm. Mass spectral analysis of the trimethylsilylated derivatives of peak III revealed a spectral pattern very similar to that published for the trimethylsilylated derivatives of 19-nor-10-keto-25-hydroxyvitamin D3. The biosynthetic 19-nor-10-keto-25-hydroxyvitamin D3 was active in the induction of differentiation of U937 cells.