Redox changes during fertilization and maturation of marine invertebrate eggs

Previous studies on sea urchin eggs indicate that activation of NAD kinase is one of the earliest Ca2+-mediated events of fertilization. The subsequently produced NADP is converted to NADPH by the pentose shunt pathway, and some of this NADPH is used by an NADPH oxidase for generation of H2O2. To examine whether these changes apply generally, we have analyzed changes in pyridine nucleotide content during meiotic maturation and fertilization in eggs from four phyla. Surprisingly, fertilization-associated increases in NAD kinase were found only in echinoid eggs. The ratio of NADPH/NADP (redox ratio) increased from 1-1.6 to 2.5-6 following fertilization of echinoid and also clam eggs. However, the ratio is already >2 for unfertilized asteroid, tunicate and echiuroid eggs, and this ratio is unaffected by fertilization. We conclude that activation of NAD kinase and shifts in pyridine nucleotide metabolism and thereby cellular redox status may have roles that vary between species. In echinoids, a major role is in providing NADPH for H2O2 production, but there may be other yet unappreciated signaling functions for this change.