Studies on the Metabolism of Hexosamine

A Kinase Activity for D-Glucosamine without Inhibition by Glucose in Rat Liver and Its Possible Identity with Glucokinase

Abstract

Phosphorylation of D-glucosamine with rat liver extracts and the effects of fasting and partial hepatectomy on its activity were studied. While the phosphorylation with brain extract is greatly inhibited by glucose and glucose-6-phosphate, the activity of liver extract has a low sensitivity to these compounds. This kinase activity as well as glucokinase [EC 2. 7. 1. 2] decreased when animals were fasted for 48 to 72h, whereas no appreciable changes were found in low K_m hexokinase [EC 2. 7. 1. 1] activity. Studies on DEAE-cellulose column chromatography showed that glucosamine was phosphorylated not only by low K_m hexokinase but also by the kinase corresponding to glucokinase. The latter enzyme peak on chromatography was reduced by fasting for 72h to half that of normal animals. The activity of phosphorylat ion of glucosamine associated with glucokinase was also reduced by the same proportion. Both activities were restored almost to those of normal liver by refeeding. Partial hepatectomy had similar effects to fasting. The activity of phosphorylation of glucosamine as well as glucokinase activity fell to 45% of the unoperated controls 48h after the operation, whereas low K_m hexokinase activity remained unchanged.
These results substantiate the finding that glucosamine is phosphorylated to an appreciable extent by hepatic glucokinase, an enzyme previously believed not to phosphorylate this hexosamine. The apparent K_m value of 8mM for glucosamine was lower than that for glucose (10-20mM). While phosphorylation of glucosamine by low K_m hexokinase was greatly inhibited by glucose, the activity associated with glucokinase was not sensitive to glucose up to 40mM.