The influence of an egg-associated peptide on energy metabolism in sea-urchin spermatozoa: the peptide stimulates preferential hydrolysis of phosphatidylcholine and oxidation of fatty acid.

A study was made of the effects of a sperm-activating peptide (SAP-I: Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly) on energy metabolism in spermatozoa of the sea urchin, Hemicentrotus pulcherrimus. The swimming activity and respiratory rate in slightly acidic seawater (pH 6.6) have been shown to be somewhat less than in normal seawater (pH 8.2). Little change occurred in sperm lipid levels during incubation in seawater at pH 6.6. The addition of SAP-I to seawater at pH 6.6 enhanced the consumption of endogenous phosphatidylcholine (PC), with no change in the levels of other lipids. SAP-I also caused increase in 14CO2 production from exogenous [1-14C]oleic acid following incubation of spermatozoa at pH 6.6. However, the stimulated levels of PC consumption and fatty acid oxidation with SAP-I at pH 6.6 did not exceed those at pH 8.2. At pH 8.2, SAP-I had no effect on PC metabolism. Activities of phospholipase A2 and fatty acid oxidation were markedly influenced by pH and increased at pH exceeding 7. SAP-I is thus concluded to stimulate sea-urchin sperm energy metabolism which depends on the oxidation of endogenous PC. It follows from these results that PC metabolism is activated following increase in the intracellular pH of spermatozoa.