The Fate of the 6a-Hydrogen of 5&holest-7-en-3@ol in the Conversion to 7-Dehydrocholesterol by Rat Liver Microsomes*

2-14C-Mevalonic acid has been incubated with a rat liver homogenate in the presence of the inhibitor frans-1,4bis(2 chlorobenzylaminoethyl) cyclohexanedihydrochloride yielding 1,7,15,22 ,26-14CS-cholesta-5,7-dien3@-01, which was reduced with lithium and ammonia to 14Cs-5ar-cholest-7en-30-01 (A’-cholestanol). Separation from endogenous cholesterol was achieved by oxidation with chromic acid, and subsequent borohydride reduction of the isolated A7-3ketone. 6/PH-5or-Cholest-7-en-3@-ol has been converted in 3% yield to 6-3H-cholesta-5,7-dien-3/3-ol by a rat liver microsomal preparation containing (frans-1,4-bis(Z-chlorobenzylaminoethyl)-cyclohexane dihydrochloride. 6ar-3HJ4C5-5oXholest-7-en-3/3-o1 has been incubated with a rat liver microsomal preparation, being converted in 5.5% yield into 14Cb-cholesta-5,7-dien-3/3-ol with loss of tritium. Less than 1% of the liberated tritium was found in isolated NADH, the remainder being isolated as 3H20. In a control experiment, synthetic NADH-4-3Hz was incubated with a microsomal preparation as before. The resulting NADH, isolated in 30% yield, contained 70% of the 3H (corrected to 100% isolation); the remainder of the radioactivity was isolated as 3HtO. Clearly then in the conversion of 5a-cholest-7-en-30-01 into 7-dehydrocholesterol, the 6cy-hydrogen is removed as a proton, and is not directly transferred to NAD+. Had NADH-3H been formed, not more than a 30% loss of 3H would have been expected during the isolation of the NADH.