Tissue-specific expression of glucokinase: identification of the gene product in liver and pancreatic islets.

The tissue distribution of glucokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) was examined by protein blotting analysis. Antibodies raised against rat liver glucokinase recognized a single protein subunit with an apparent Mr of 56,500 on nitrocellulose blots of cytosol protein from liver, separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A protein of identical electrophoretic mobility was detected by immunoblotting of cytosol protein from pancreatic islets. Hepatic glucokinase and the immunoreactive islet product bound to and were eluted from DEAE-cellulose at the same ionic strength. Glucokinase was displayed as a set of two spots with apparent pI values of 5.54 and 5.64 by immunoblotting after two-dimensional gel electrophoresis. The two isoforms appeared equally abundant in liver extract, whereas the component with a pI of 5.64 was predominant in islets. By quantitative immunoblotting, glucokinase was estimated to represent 0.1% of total cytosol protein in liver and 1/20th as much in islets. The glucokinase activity of both liver and islet cytosols was suppressed by the antibodies to hepatic glucokinase. Immunoblotting of cytosol protein from intestinal mucosa, exocrine pancreas, epididymal adipose tissue, kidney, brain, and spleen failed to reveal the glucokinase protein. Thus, significant expression of the glucokinase gene appears restricted to the liver and pancreatic islets.