CHANGES IN ACTIVITIES OF THYMIDYLATE SYNTHASE AND DIHYDROFOLATE REDUCTASE IN SEA URCHIN EGGS AFTER FERTILIZATION

Abstract

Thymidylate synthase [EC 2.1.1.6] activity in sea urchin [Clypeaster japonicus, Anthocidaris crassispina, Pseudocentrotus depressus and Hemicentrotus pulcherrimus] eggs increases just after fertilization and decreases 30 min later. Then, cyclic variation in the activity occurs in association with the cleavage cycle. Dihydrofolate reductase [EC 1.5.1.3] activity in fertilized eggs is almost the same as in unfertilized eggs and shows no marked change within 3 h after fertilization. Aminopterin, an analog of dihydrofolate, inhibits dihydrofolate reductase, and arrests cleavage. On incubation in sea water containing aminopterin (20-100 .mu.M) from the time of fertilization, the development of Clypeaster and Pseudocentrotus eggs was arrested at the 32-64 cell stage, and that of Anthocidaris eggs was arrested at the morula stage. Dihydrofolate (100 .mu.M) counteracts the inhibitory effect of aminopterin on egg cleavage. Thymidine at concentrations above 10 .mu.M also prevents inhibition by aminopterin. Other deoxyribonucleosides at concentrations of 10-100 .mu.M do not affect inhibition of egg cleavage by aminopterin. Deoxyadenosine at concentrations above 5 mM inhibits egg cleavage, but other deoxyribonucleosides have no effect.