Properties of adenylate cyclase activity during early sea urchin development

Abstract

The total adenylate cyclase activity in homogenates of eggs of the sea urchins Strongylocentrotus purpuratus and Lytechinus pictus was assayed in vitro and found to remain constant in eggs before and at intervals after fertilization. In S. purpuratus egg homogenates virtually all of the enzyme activity was sedimented by centrifugation at 20 000 g. The enzyme specific activity in the 20 000 g pellet remained unchanged at each point through first cleavage, though it was several-fold higher than in the whole homogenate. The adenylate cyclase from both fertilized and unfertilized eggs was maximally active in vitro when assayed with 10 mM MgSO 4 and 10 mM NaF at pH 8 using 0.2 mM AMP-PNP (an ATP analog) as the substrate. Sucrose density gradient centrifugation of egg homogenates showed that adenylate cyclase activity was present in fractions which sedimented at a variety of densities. The adenylate cyclase specific activity in cortices isolated by the method of Sakai [10] from eggs at first cleavage was 4- to 6-fold higher than in unfertilized egg cortices. The increased enzyme activity in egg cortices at first cleavage suggests that adenylate cyclase-containing membranes may become localized within the egg cortex after fertilization.