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.