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

Abstract

Membrane enriched fractions of eggs show changes in relative lipid and acylglyceride fatty acid content at maturation and fertilization, when many critical membrane changes occur. Many of the changes in lipids seen at fertilization in sea urchins are reversed from those seen at maturation in starfish. Maybe a special permissive lipid content appears at maturation and disappears at fertilization. Mycostatin alters the lipid content of unfertilized eggs, prevents fertilization, blastula cell aggregation and normal development, with decreasing susceptibility with age, suggesting altered cell surfaces during development. Membrane enriched fractions also show changes in enzyme specific activity at fertilization. Attempts were made to localize these enzymes in various surface components (membrane or associated coats or adherent cortex) and several fractions were separated. Egg surfaces are complex and therefore difficult to study. DTT or pronase treatment, for example, changes the activity of G6PD in the surface of whole eggs, with further changes occurring in such eggs following fertilization. Increases in G6PD activity in ghosts and supernate of fertilized eggs appear due to isoenzyme changes instead of one enzyme changing compartment. Isozyme patterns of G6PD extracted from pellets differ from those of supernates. Patterns also differ following fertilization or partial activation by ammonia or calcium ionophore. Some isozymes seem to be breakdown products of larger forms, and can be produced by treatment with papain, DTT, or urea. Isozyme studies support a dynamic model of the egg surface. Surface changes may occur enzymatically, by insertion of vesicles, or by release of material from vesicles.