Enzymatic Studies on Metabolic Adaptation of Hexose Monophosphate Shunt in Rat Liver*

The mechanism of the alteration in the enzyme activity was investigated concerning the hexose monophosphate shunt in rat liver. Refeeding of unfed rats on high-carbohydrate diet caused the increases of specific activity of the enzymes in the supernatant fraction; G6PDH (+1,280%), 6PGL (+110%) and 6PGDH (+172%) without increase in LDH. This effect was specific for liver among various tissues. The effect on the dehydrogenases was hardly caused by a high-fat diet and was inhibited by the administration of 8-azaguanine. The increased activities of the three enzymes were stable against protein purification procedures. There was no significant difference in kinetic properties of the enzymes between unfed and refed rats. Participation of activators or inhibitors and the changes in enzyme stability and intracellular distribution in this phenomenon was not detected. Titration of G6PDH with NADPH suggested 30-fold increase of the enzyme in refed livers. It may he concluded that the elevated enzyme activities in this phenomenon are caused by the increase in enzyme protein. The nature of this adaptation was discussed.