CHANGE IN THE ACTIVITY OF PYRUVATE DEHYDROGENASE COMPLEX IN SEA URCHIN EGGS FOLLOWING FERTILIZATION

The activity of the pyruvate dehydrogenase complex in sea urchin eggs is localized in the crude mitochondrial fraction. The activity of the enzyme complex in the intact mitochondrial fraction of unfertilized eggs is too low to be estimated and is enhanced upon fertilization with a 5-min lag period. The activity of the enzyme complex in unfertilized eggs is enhanced by Ca2+ at concentrations between 5 × 10-5 M and 10-3 M. The activity in fertilized eggs is blocked after incubation with 2 mM ATP, and the block of the activity is also released by Ca2+ . The blockage of the enzyme complex activity is accompanied by phosphorylation of proteins, and release of the block by Ca2+ is concomitantly followed by the dephosphorylation of proteins in the mitochondrial fraction. The enzyme complex in unfertilized eggs will be assumed to be the one inhibited by phosphorylation. The enzyme complex will be activated upon fertilization as a consequence of the dephosphorylation, that is caused by the increase in intracellular concentration of Ca2+ .