Phosphatidylinositol metabolism during fertilization in the sea urchin egg

Fertilization of the sea urchin egg results in a transient decline in the amount of phosphatidylinositol (PI) to a level equal to about 50% of that present in the unfertilized egg. This response begins as early as 15 seconds after insemination. The level of PI reaches a minimum at 30 seconds post-insemination, and returns to the original value between 2 and 5 min later. Pulse labelling studies with³²PO₄ and ³H]-inositol showed that the incorporation of these two isotopes into 1-(3-sn-phosphatidyl)-L-myoinositol 4,5-biphosphate PtdIns(4,5)P₂] increased as much as 50% within one minute after insemination. This suggests that at least part of the reduction in PI levels represents the phosphorylation of PI to form PtdIns(4,5)P₂. We also found that the production of ³H]-labelled 1D-myoinositol 1,4,5 triphosphate Ins(1,4,5)P₃] present in the trichloroacetic acid (TCA) soluble fraction of eggs increased over five-fold during the first 10 min post insemination. The temporal correlation between the early burst of PtdIns(4,5)P₂ and Ins(1,4,5)P₃ formation and the transient increase in intracellular free calcium known to occur in the fertilized egg suggest that the production of PtdIns(4,5)P₂ and ultimately Ins(1,4,5)P₃ may be associated with calcium mobilization within the egg.