Hormonal activation of phospholipid methyltransferase in the Leydig cell

Hormonal stimulation of purified rat Leydig cells with hCG caused a 2-fold increase in phospholipid methyltransferase activity within 10 min. This effect of gonadotropin was concentration dependent, with maximal activation of methyltransferase at 10(-12) M hCG, and was reproduced by 8-bromo-cAMP. Hormonal activation of phospholipid methyltransferase caused an increase in the maximum velocity of the enzyme, without changing the Km for S-adenosylmethionine. Analysis of the pattern of radiolabeled phospholipids from Leydig cells showed that the incorporation of methyl groups and the increase in phospholipid methylation stimulated by 8-bromo-cAMP occurred mainly in phosphatidyl-N-monomethylethanolamine. The rapid and highly sensitive increase in phospholipid methylation in hCG-stimulated Leydig cells is consistent with a role of methyltransferase in the hormonal activation mechanism probably at a step subsequent to cAMP production.