Glucose-Regulated Proinsulin Processing in Isolated Islets From Rat Pancreas

We demonstrated previously that the conversion rate of proinsulin to insulin in pancreatic islets progressively increased after prolonged prior exposure to glucose (11 mM) and that this effect could be blocked by cycloheximide. This study was designed to characterize further the time course and regulation of the proinsulin conversion process. The effects of prior exposure to glucose on proinsulin conversion were dose dependent (K_m, ∼7 mM glucose) and time dependent, taking ∼3 h to reach the maximum rate. Glucose added at or after the subsequent [³H]leucine pulse was ineffective. Mannoheptulose, added during a 3-h exposure with glucose (11 mM), prevented glucose-induced activation of the proinsulin conversion process. L-Leucine (20 mM) was as effective as 11 mM glucose in activating conversion, whereas 2-α-ketoisocaproic acid (20 mM) or phorbol ester (50 nM) had little effect. Activation of proinsulin conversion by a 24-h exposure to glucose (11 mM) was reversed by a subsequent 3-h prior exposure to cycloheximide. α-Amanitin, an inhibitor of mRNA synthesis, did not influence the glucose-induced activation of proinsulin conversion when present during a 3-h exposure to glucose; however, it completely inhibited glucose-stimulated conversion when present during 24 h exposure. Results suggest that activation of the proinsulin conversion process is regulated by glucose metabolism rahier than the glucose molecule per se and that other, but not all, secretagogues are effective. Conversion may require prior synthesis of a pool of converting enzyme(s) or other regulatory proteins whose turnover is relatively rapid (∼33 h) and whose mRNA is more stable (to 24 h).