Mammalian sperm contain a Ca2+-sensitive phospholipase C activity that cangenerate InsP(3) from PIP2 associated with intracellular organelles

We have previously described a phospholipase C (PLC) activity in mammalian sperm cytosolic extracts. Here we have examined the Ca2+ dependency of the enzyme, whether there is enough in a single sperm to account for Ca2+ relea se at fertilization, and finally where in the egg is the phosphatidyl 4,5-b isphosphate, the substrate for the enzyme. As for all PLCs examined so far in vitro, we found that the boar sperm PLC activity was Ca2+ dependent. Spe cific activity increased when Gee Ca2+ levels were micromolar. However, eve n at nanomolar free Ca2+ concentration the boar sperm PLC activity was cons iderable, being two orders of magnitude greater than PLC activities in othe r tissues. We calculated that PLC activity of a single boar sperm in a mamm alian egg is enough to generate 400 nM inositol 1,4,5-trisphosphate (InsP(3 )) in 1 min, which may be sufficient to account for the observed Ca2+ chang es in an egg at fertilization. We fractionated sea urchin egg homogenate an d examined the ability of boar sperm extract to generate InsP(3) from these fractions. The sperm PLC activity triggered InsP(3) production from a PIP2 -enriched nonmicrosomal egg compartment that contained yolk platelets. We p ropose that this sperm PLC activity, which is active at nanomolar Ca2+ leve ls and hydrolyzes PIP2 from intracellular membranes, could be involved in t he Ca2+ changes observed at fertilization. (C) 2000 Academic Press.