EXPRESSION OF GLUCOKINASE IN GLUCOSE-UNRESPONSIVE HUMAN FETAL PANCREATIC ISLET-LIKE CELL CLUSTERS

Glucokinase (GK) is the glucose sensor in the adult beta-cell, resulti ng in fuel for insulin synthesis and secretion. Defects in this enzyme in the beta-cell are responsible for the genetic disorder maturity-on set diabetes of the young, with the beta-cell being unable to secrete insulin appropriately when challenged with glucose. The human fetal be ta-cell is also unable to secrete insulin when exposed to glucose, but whether GK is present and functional in this developing cell is unkno wn. To determine the expression of GK in human fetal pancreatic tissue , cytosolic protein was extracted from human fetal islet-like cell clu sters (ICCs) at 17-19 weeks gestation and examined for protein content and enzyme activity. On Western blots, a single band corresponding to GK was seen at 52 kDa, and this was similar to that obtained from hum an adult islets. The maximal velocity (V-max) of GK was less in fetal ICCs than that in adult islets (8.7 vs. 20.7 nmol/mg protein h); simil ar K-m values were found in both ICCs and islets. No attempt was made to determine which cells in an ICC contained GK. Glucose utilization w as determined radiometrically; the V-max of the high K-m component was less in ICCs than in islets (31.3 mu mol/ICC . h us. 101.4 pmol/islet . h). Culture of ICCs for 3-7 days in medium containing 11.2 mmol/L g lucose resulted in a 3.7-fold increase in the V-max of GK and a 1.8-fo ld increase in glucose utilization. These enhanced activities of gluco se phosphorylation and glycolysis, however, did not lead to the beta-c ell being able to secrete insulin when exposed to glucose. In conclusi on, glucokinase is present and functional in human fetal ICCs, but the inability of the human fetal beta-cell to secrete insulin in response to an acute glucose challenge is not due to immaturity of this enzyme .