Different Ca2+-releasing abilities of sperm extracts compared with tissue extracts and phospholipase C isoforms in sea urchin egg homogenate and mouse eggs

A soluble phospholipase C (PLC) from boar sperm generates InsP(3) and hence causes Ca2+ release when added to sea urchin egg homogenate. This PLC acti vity is associated with the ability of sperm extracts to cause Ca2+ oscilla tions in mammalian eggs following fractionation. A sperm PLC may, therefore , be responsible for causing the observed Ca2+ oscillations at fertilizatio n. In the present study we have further characterized this boar sperm PLC a ctivity using sea urchin egg homogenate. Consistent with a sperm PLC acting on egg PtdIns(4,5)P-2, the ability of sperm extracts to release Ca2+ was b locked by preincubation with the PLC inhibitor U73122 or by the addition of neomycin to the homogenate. The Ca2+-releasing activity was also detectabl e in sperm from other species and in whole testis extracts. However, activi ty was not observed in extracts from other tissues. Moreover recombinant PL C beta 1, -gamma 1, -gamma 2, -delta 1, all of which had higher specific ac tivities than boar sperm extracts, were not able to release Ca2+ in the sea urchin egg homogenate. In addition these PLCs were not able to cause Ca2oscillations following microinjection into mouse eggs. These results imply that the sperm PLC possesses distinct properties that allow it to hydrolyse PtdIns(4,5)P-2 in eggs.