Abstract
Glucokinase (GK) is the glucose sensor in the adult β-cell, resulting in fuel for insulin synthesis and secretion. Defects in this enzyme in the β-cell are responsible for the genetic disorder maturity-onset diabetes of the young, with the β-cell being unable to secrete insulin appropriately when challenged with glucose. The human fetalβ -cell is also unable to secrete insulin when exposed to glucose, but whether GK is present and functional in this developing cell is unknown. To determine the expression of GK in human fetal pancreatic tissue, cytosolic protein was extracted from human fetal islet-like cell clusters (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 human adult islets. The maximal velocity (Vmax) of GK was less in fetal ICCs than that in adult islets (8.7 vs. 20.7 nmol/mg protein·h); similar Km values were found in both ICCs and islets. No attempt was made to determine which cells in an ICC contained GK. Glucose utilization was determined radiometrically; the Vmax of the high Km component was less in ICCs than in islets (31.3 pmol/ICC·h vs. 101.4 pmol/islet·h). Culture of ICCs for 3–7 days in medium containing 11.2 mmol/L glucose resulted in a 3.7-fold increase in the Vmax of GK and a 1.8-fold increase in glucose utilization. These enhanced activities of glucose phosphorylation and glycolysis, however, did not lead to the β-cell being able to secrete insulin when exposed to glucose. In conclusion, glucokinase is present and functional in human fetal ICCs, but the inability of the human fetal β-cell to secrete insulin in response to an acute glucose challenge is not due to immaturity of this enzyme.
Insulin synthesis, secretory competence, and glucose utilization are sensitized by transgenic yeast hexokinase.