Depression of human sperm motility by inhibition of enzymatic methylation

Abstract

Alteration of membrane fluidity during enzymatic methylation of membrane phosphatidylethanolamine (PE) and neutralization of negative charges of membrane proteins due to methylation of carboxyl groups may contribute to sperm motility. Therefore, enzymatic phospholipid methylation and carboxymethylation, and the consequences of their inhibition on motility, were studied using human sperm. These studies gave the following results. Human sperm homoganates contained two phospholipid N-methyltransferases (PMT) which converted PE to phosphatidylcholine (PC) in the presence of S-adenosylmethionine (SAM). The first PMT converted PE to phosphatidyl- N-methylethanolamine (PME). In had a K m of 4.0 μM and a pH optimum of 8.0. The second PMT converted PME to phosphatidyl- N, N-dimethylethanolamine and PC. It had a K m of 71μM and a pH optimum of 10.0. Spermatozoa also contained protein carboxymethylase (PCM) and methyl aceptor protein (MAP). The intracellular levels of S-adenosylhomocysteine (SAH), an inhibitor of SAM-mediated methylations, were increased by adding adenosine (100 μM), l-homocysteine thiolactone (L-HCT, 10 μM), and erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA, 10 μM), an inhibitor of adenosine deaminase, to human sperm ejaculates that had been diluted with sodium phosphate buffer at pH 7.4 and 25°. The motility index of each sperm suspension was determined every hour for 4 hr. In the presence of the mixture of adenosine, L-HCT and EHNA, the motility index was depressed by 57%. Under similar conditions, phospholipid methylation was depressed by 48%. Similar experiments were also conducted in the presence of 3-deazaadenosine (Deaza, 80 μM), a selective inhibitor of SAH hydrolase. In the presence of adenosine and L-HCT, Deaza depressed the motility index by 60% and phospholipid methylation by 86%. The potencies of SAH in the inhibition of phospholipid methylation and protein carboxymethylation in sperm homogenates had the following order: PMT I > PCM > PMT II. These observations indicate that the PMT system and/or the PCM-MAP system play a significant role in the regulation of human sperm motility.