Methylating Activity in Ovarian Membranes Changes During the Estrous Cycle

The aim of this study was to characterize the methylation system of phosphatidylethanolamine (PE) in the ovarian membranes and identify their possible role on the ovarian physiology. The presence of two methylating activities was found, by using S-adenosyl-L-[methyl-³H] methionine (SAM) as methyl donor. One of them (Met I) uses PE as substrate, and the other one, (Met II), catalyses the two successive methylations of phosphatidyl-N-monomethylethanolamine (PME) to phosphatidylcholine (PC). Met I shows a Km value of 5.71 µM and a Vm of 1.53 pmol/mg protein · min, working at pH 6.5, which seems to be the optimum. Met II exhibits low affinity for SAM, a Km of 50 µM, a Vm of 1.2 pmol/mg protein · min and it works an optimum pH of 9.5. Those enzymatic properties are in agreement with the amount of monomethylated and trimethylated products found working at pH 6.5 and 9.5, respectively. In order to investigate whether this enzyme system is affected or not by the hormonal environment influencing the ovary during the estrous cycle, the methylating activity was measured at its different stages. Both methylating activities were induced on proestrus but only Met I on diestrus. The total methylating activity increases when the ovarian membranes were obtained from rats injected with pregnant mare serum gonadotrophin (PMSG). Those results suggest that phospholipid methylation could be regulated by the hormonal environment during the estrous cycle.

Key words

Phospholipid Methylation - Ovary - Methyl-transferases - Estrous Cycle