Glycerol kinase activity and glycerol kinase-3 gene are up-regulated by acclimation to 5 °C in diapause eggs of the silkworm, Bombyx mori

With initiation of diapause, glycogen is converted into sorbitol and glycerol in eggs of the silkworm Bombyx mori. At diapause termination promoted by incubation at 5 °C, sorbitol and glycerol are utilized. Although sorbitol utilization is triggered by NAD-sorbitol dehydrogenase induced by acclimation to 5 °C, the initial enzyme utilizing glycerol remains unclear. In this study, we detected glycerol kinase activity in diapause-terminated eggs and then characterized its properties; maximal activity was seen at pH 8.5-9.0, and Km values for glycerol and ATP were 0.32 and 0.095 mM, respectively. In diapause eggs continuously kept at 25 °C, the activity was almost negligible. However, activity was seen after chilling for 60 days and thereafter increased when the eggs were exposed to 5 °C after 2 days post-oviposition, indicating that glycerol kinase is a rate-limiting enzyme in glycerol utilization. We then cloned cDNAs encoding glycerol kinase-1, -2 and -3 from B. mori. Only gene expression of glycerol kinase-3 was up-regulated in diapause eggs exposed to 5 °C, indicating that glycerol kinase activity is regulated via transcription of the glycerol kinase-3 gene.