NAD-dependent sorbitol dehydrogenase activity in relation to the termination of diapause in eggs of Bombyx mori

Abstract

Some properties of NAD-dependent sorbitol dehydrogenase (NAD-SDH) were determined in crude extracts from chilled, diapause eggs of Bombyx mori. Optimum pH was approx. 8.8 and the apparent K m values for NAD and sorbitol were 0.20 mM and 136 mM, respectively. In the reaction, sorbitol was stoichiometrically converted to fructose, but glycerol and mannitol were not oxidized. During early embryonic development (up to 2 days after oviposition) NAD-SDH activity was almost undetectable in both non-diapause and diapause types of eggs. The activity remained at this level in the diapausing eggs while it increased with age in the non-diapausing ones. After chilling the diapause eggs for 3 months, NAD-SDH activity began to increase gradually and then rose markedly. Also, a rapid increase in NAD-SDH activity was observed in eggs which were treated with HCl to break diapause quickly. These results together with the changing patterns in sorbitol concentrations in the eggs suggest that NAD-SDH is a key enzyme for sorbitol degradation at the termination of diapause in eggs of B. mori. The biochemical significance of NAD-SDH is also discussed in relation to the pathway of glycogen synthesis from sorbitol.