Regulation of the tricarboxylic acid cycle in sea urchin eggs and embryos

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Abstract
Contents of the tricarboxylic acid (TCA) cycle intermediates, coenzyme A (CoA) derivatives, and carnitine derivatives were estimated in the developing eggs and embryos of the sea urchin, Anthocidaris crassispina. Levels of the TCA cycle intermediates in unfertilized eggs were similar. The level of malate increased remarkably after fertilization and reached a plateau just before hatching out. Slight elevation in the level of citrate was also observed during early development. These suggest that metabolism of citrate and malate is altered during development. Long‐chain fatty acyl‐CoA was accumulated in unfertilized eggs. Its level decreased after fertilization, then gradually increased until the late gastrula stage and turned to decline. Free CoA level increased after fertilization and reached a peak around the gastrula stage, and thereafter it decreased, while short‐chain acyl‐CoA level decreased gradually after fertilization. Crude citrate synthase from sea urchin unfertilized and fertilized eggs was inhibited by palmitoyl‐CoA, though malate dehydrogenase was not inhibited. The palmitoyl‐CoA‐caused inhibition of citrate synthase was also relieved by spermine and spermidine. In view of the inhibition of citrate synthase by palmitoyl‐CoA at concentrations in unfertilized eggs, the enzyme is probably inhibited in unfertilized eggs by long‐chain acyl‐CoA and released from the inhibited state by the decrease in the level of long‐chain acyl‐CoA and the increase in the level of polyamines following fertilization.