Ebb and Tide of Glucokinase

his journal’s editors have asked me to comment on the paper in this issue by Jetton et al. entitled "Substrate induced nuclear export and peripheral compartmentalization of hepatic glucokinase correlates with glycogen deposition." The investigations of these authors exemplify the remarkable recent trend in studies of intermediary metabolism paying careful attention to the precise intracellular location of biochemical processes including also those that are conventionally assumed to occur in the aequeous cytoplasmic compartment. The investigative history of the glucokinase enzymemthe topic of the paper editorialized heremstrikingly illustrates the slow pace of evolution in studies of this particular aspect of intermediary metabolism, starting perhaps with the pioneering concepts of Paul Srere in the sixties [1]. Glucokinase was long considered a typical soluble cytosolic enzyme. Sidney Weinhouse and Alberto Sols discovered the enzyme in the high speed supernatant of liver extracts with negligible residue in the particulate fraction [2,3]. This distribution contrasted with that of glucose-6phosphatase which was associated with the microsomes. A striking intraacinar gradient of glucokinase exhibiting significantly higher activities in the pericentral as compared to the peripheral zones was discovered in the late seventies [4]. It is of interest that glucose-6-phospahatase has an intraacinar gradient opposite to that of glucokinase. The kinetic characteristics of the enzyme, its S0.5 of about 8.0 mM, ATP Km of about 0.3 mM and Hill coefficient of about 1.7 explained its optimal operation in the aequeous cytosolic compartment of hepatocytes where physiological glucose levels of 5 to 8 mM approximate those of plasma and ATP2-, the second substrate, is about 2.5 mM [5]. Consequently, a soluble glucokinase could operate close to its inflection point of about 4.0 mM where it is most responsive to changes of glucose levels. Since the enzyme is not directly controlled by glucose-6phosphate as feedback inhibitormmore about this laterm regulation of its activity was initially thought to be primarily due to alteration of protein synthesis stimulated by insulin. After glucokinase was discovered in pancreatic islets the conceptualization of the enzyme as cytoplasmic beta-cell glucose sensor followed the general outline of its biochemistry in hepatocytes except that its expression was considered to be controlled directly by glucose rather than by insulin [5].