Cholesterol biosynthesis by rat liver microsomes: Concerning C-5 double bond introduction

Abstract

The ionic mechanism of dehydrogenation resulting in double bond formation is thought to proceed via the abstraction of a hydride ion and a proton from neighboring carbon atoms. The applicability of such a mechanism to the conversion of 5α-cholest-7-en-3β-ol to cholesta-5,7-dien-3β-ol by a microsomal rat liver acetone powder preparation [16] was investigated. The synthesis of the required substrate 5α- 3H]cholest-7-en-3β-ol is described. Incubation of [5α- 3H; 14C 5]-cholest-7-en-3β-ol with the enzyme preparation in the presence of added AY 9944; NAD + and air gave [ 14C 5]-cholesta-5,7-dien-3β-ol. The tritium abstracted from the substrate was found in the water and the recovered NADH contained little tritium. A similar incubation was carried out with the previously prepared [6α- 3H; 14C 5]-5α-cholest-7-en-3β-ol [15]. Again in this instance the abstracted tritium atom was found in the water while the recovered NADH contained an insignificant amount of tritium. The results indicate that both the 5α- and 6α-hydrogen (tritium) atoms removed from the 7-ene in the conversion to the 5,7-diene under the conditions employed are ultimately transferred to the water of the medium. The route by which these hydrogen atoms are abstracted is not clear and requires further study.