Relationship between ATP level and respiratory rate in sea urchin embryos

In sea urchin embryos, the rate of respiration, as a result of electron transport through the mitochondrial respiratory chain, was enhanced after hatching without any change in the intrinsic capacity of electron transport in mitochondria. The increase in respiratory rate after hatching was accompanied by an evident decrease in intracellular adenosine triphosphate (ATP) concentration without any change in intracellular levels of adenosine diphosphate (ADP) and adenosine monophosphate (AMP). Adenosine triphosphate is proposed to fortify acceptor control of respiration at high concentrations and to reduce the respiratory rate even in the presence of ADP, the acceptor. The relationships between the respiratory rate and intracellular ATP concentration in embryos were the same as those in mitochondria isolated from embryos, obtained in the presence of ADP at the same concentration as in the embryos. Probably, the respiratory rate is enhanced after hatching because of the decrease in the level of ATP. In embryos kept in a medium containing adenosine, intracellular ATP concentration increased especially after hatching, without any change in the ADP level, and the respiratory rate after hatching was made as low as the rate expected, based on the relationships obtained on isolated mitochondria. The respiratory rate in embryos probably depends on intracellular ATP concentration, irrespective of the developmental stage in early development.

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

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.

The cytochrome system of sea urchin eggs and embryos

The cytochrome system in eggs and embryos of the sea urchin, Hemicentrotus pulcherrimus, was investigated. Difference spectra of the mitochondrial fraction demonstrated the presence of a complete cytochrome system in unfertilized eggs. Cytochrome levels and the activities of respiratory enzymes were measured in crude extracts of eggs both before and after fertilization. Unfertilized eggs contained cytochromes aa3, b, and c + c1 in a ratio of 1.0:1.8:0.7. Gastrulae contained almost the same amount of cytochromes aa3 and b as unfertilized eggs. However, the amount of cytochrome c + c1 in gastrulae was 1.5 times greater than that in unfertilized eggs. The activity of cytochrome oxidase remained unchanged during development. No cytochrome oxidase inhibitor was found in unfertilized eggs. Both antimycin A-sensitive and insensitive NADH-cytochrome c reductase activities increased during development. The activity of succinate-cytochrome c reductase increased during early development, reached a temporary plateau, and then declined at the pluteus stage. These results are discussed in relation to the increase of respiration during early development.