Effects of Diet and Insulin on Glucose-Adenosine Triphosphate Phosphotransferases of Rat Liver

Abstract

Recent studies from our laboratory have disclosed that rat liver contains a glucose-adenosine triphosphate phosphotransferase which differs from the hitherto described conventional hexokinases in having a low aflinity for glucose, with a K, of 0.01 to 0.02 M (1). The activity of this enzyme falls to low levels in fasting and alloxan diabetes (2), and its physiological role in the control of hepatic glucose metabolism has been pointed out (l-4). A high blank, owing in part to the presence of endogenous glucose in the liver extracts used for assay of this enzyme, left uncertain whether liver also contains small amounts of an enzyme with a low K, for glucose. However, in recent preliminary communications, Walker (5) and Walker and F&o (6) reported kinetic data indicating the presence of both low K, and high K, glucokinases in guinea pig and rat liver, and Viiiuela, Salas, and Sols (7), in another preliminary report, also established the presence in rat liver of a hexokinase with a low glucose K, in addition to the previously reported high K, enzyme. The latter investigators separated the two enzymes by ammonium sulfate fractionation, and also described a simple assay procedure for their separate determination in crude tissue fractions. This procedure consists in determining glucose 6-phosphate formation spectrophotometrically at two different glucose concentrations, and correcting for endogenous ATP-dependent TPN+ reduction by running the reaction in the presence of the hexokinase inhibitor, acetylglucosamine. For convenience we designate the low K, enzyme hexokinase, and the high K, enzyme, glucokinase (cj. (7)). By this assay method, we have conducted further studies of dietary and hormonal effects on both hexokinases. The results confirm those of Vifiuela et al. (7) and Walker (5, 6) in demonstrating the presence of a hexokinase in rat liver with a low K, for glucose, and they extend and amplify our previous findings as well as those of Viiiuela et al. (7) in showing that it is only the high K, glucokinase which changes in activity with dietary and hormonal alterations.