The Possible Roles of Membrane Organization in the Activity of Androgen Biosynthetic Enzymes Associated with Normal or Tumorous Mouse Leydig Cell Microsomes*

Abstract

The enzymes involved in conversion of pregnenolone to testosterone in Leydig cell tumors showed a wide distribution among smooth endoplasmic reticulum (SER), rough endoplasmic reticulum (RER), and cytosol, while these enzymatic activities in normal testes were associated primarily with smooth endoplasmic reticulum. Progesterone, used as a substrate in the presence of an NADPH-generating system, was metabolized to androstenedione and finally to testosterone by microsomes from some strains of tumor which did not form testosterone from exogenous labeled androstenedione. Treatment of microsomal membranes from normal testes with 0.1 M Ca2+ and Mg2+ caused a marked decrease in 17.beta.-dehydrogenase activity, measured as conversion of exogenous [3H]androstenedione to [3H]-testosterone, without serious effects on activities of 3.beta.-ol-dehydrogenase or 17.alpha.-hydroxylase. Studies of initial velocity kinetics showed that treatment with Mg2+ resulted in a marked reduction in affinity of androstenedione for 17.beta.-dehydrogenase while the Vmax was the same as in untreated microsomes. Experiments using [14C]progesterone and [3H]androstenedione simultaneously as substrates demonstrated that treatment with mg2+ made it more difficult for exogenous [3H]androstenedione to reach the active site of 17.beta.-ol-dehydrogenase than [14C]androstenedione formed in the microsomal membrane from [14C]progesterone. Microsomal proteins were more easily solubilized and 3.beta.-ol-dehydrogenase was more severely influenced by Mg2+ ion in tumor membranes than in normal microsomes.