Malate-aspartate cycle as an effective hydrogen shuttle at the termination of diapause in the eggs of Bombyx mori

Abstract

The enzymes, which constitute the malate-aspatate cycle as a hydrogen shuttle, were examined in the eggs of Bombyx mori. Glutamate-oxaloacetate transaminase (GOT) and malate dehydrogenase (MDH) were found in both cytosolic and mitochondrial compartments of the eggs of silkworms and had specific kinetic properties. The activities of these enzymes were correlated with embryonic development and attained maximum levels at larval hatching. The activities of mitochondrial GOT and MDH increased in diapause eggs which were chilled at 5°C for more than 20 days. A further and rapid increase in the activities of mitochondrial GOT and MDH was induced by HCl-treatment of the chilled eggs. However, activities of the mitochondrial enzymes in non-diapause eggs were not influenced by chilling. The reconstruction experiments with intact mitochondria showed the occurrence of the malate-aspartate cycle in the silkworm eggs. The functional significance of the malate-aspartate cycle as a hydrogen shuttle is discussed in relation to NAD-sorbitol dehydrogenase at the termination of diapause in silkworm eggs.