DELTA-5-3-BETA-HYDROXYSTEROID ACYL TRANSFERASE-ACTIVITY IN THE RAT-BRAIN

Pregnenolone and dehydroepiandrosterone accumulate in brain as sulfate and fatty acid esters and unconjugated steroids. The steroid fatty acid ester-synthesizing activity was investigated in rat brain microsomes. Endogenous fatty acids in the microsomal fraction were used for the esterification of steroids. The enzyme system had a pH optimum of 4.5 in acetate buffer with [H-3]dehydroepiandrosterone as substrate. The apparent Km was 9.2 +/- 3.1 x 10(-5) M and V(max) was 18.6 +/- 3.4 nmol/h/mg protein (mean +/- SEM). The inhibition constants of pregnenolone and testosterone were 123 and 64-mu-M, respectively. Results were compatible with a competitive type of inhibition. A high level of synthetic activity was found in the brain of 1- to 3-week-old male rats, which rapidly decreased with aging. Saponification of purified [H-3]pregnenolone esters yielded pregnenolone and a mixture of palmitate, oleate, linoleate, stearate, and myristate as the predominant fatty acids. Contrasting with the high rates of esterification of several radioactive DELTA-5-3-beta-hydroxysteroids or 17-beta-hydroxysteroids, no fatty acid esters of either cholesterol, epitestosterone (with a hydroxyl group at position C-17-alpha), or corticosterone (with hydroxyl groups at C-21 and C-11-beta) were formed in the same incubation conditions.