Changes in Enzyme Activities and Lipid Content of Echinoderm Egg Membranes, at Maturation and Fertilization

Membrane enriched fractions of eggs show changes in relativelipid and acylglyceride fatty acid content at maturation andfertilization, when many critical membrane changes occur. Manyof the changes in lipids seen at fertilization in sea urchinsare reversed from those seen at maturation in starfish. Maybea special permissive lipid content appears at maturation anddisappears at fertilization. Mycostatin alters the lipid contentof unfertilized eggs, prevents fertilization, blastula cellaggregation and normal development, with decreasing susceptibilitywith age, suggesting altered cell surfaces during development.Membrane enriched fractions also show changes in enzyme specificactivity at fertilization. Attempts were made to localize theseenzymes in various surface components (membrane or associatedcoats or adherent cortex) and several fractions were separated.Egg surfaces are complex and therefore difficult to study. DTTor pronase treatment, for example, changes the activity of G6PDin the surface of whole eggs, with further changes occurringin such eggs following fertilization. Increases in G6PD activityin ghosts and supernate of fertilized eggs appear due to isoenzymechanges instead of one enzyme changing compartment. Isozymepatterns of G6PD extracted from pellets differ from those ofsupernates. Patterns also differ following fertilization orpartial activation by ammonia or calcium ionophore. Some isozymesseem to be breakdown products of larger forms, and can be producedby treatment with papain, DTT, or urea. Isozyme studies supporta dynamic model of the egg surface. Surface changes may occurenzymatically, by insertion of vesicles, or by release of materialfrom vesicles.