Enzymes in pancreatic islets that use NADP(H) as a cofactor including evidence for a plasma membrane aldehyde reductase.

Recent evidence of a pyruvate malate shuttle capable of transporting a large amount of NADPH equivalents out of mitochondria in pancreatic islets suggests that cytosolic NADP(H) plays a role in beta cell metabolism. To obtain clues about these processes the activities of several NADPH-utilizing enzymes were estimated in pancreatic islets. Low levels of pyrroquinolone quinone (PQQ) and low levels of enzyme activity that reduce PQQ were found in islets. Low activities of palmitoyl-CoA and stearoyl-CoA desaturases were also detected. Significant activities of glutathione reductase, aldose reductase (EC.1.1.1.21) and aldehyde reductase (EC.1.1.1.2) were present in islets. Potent inhibitors of aldehyde and aldose reductases inhibited neither glucose-induced insulin release nor glucose metabolism in islets indicating that these reductases are not directly involved in glucose-induced insulin reaction. Over 90% of aldose reductase plus aldehyde reductase enzyme activity was present in the cytosol. Kinetic and chromatographic studies indicated that 60-70% of this activity in cytosol was due to aldehyde reductase and the remainder due to aldose reductase. Aldehyde reductase-like enzyme activity, as well as aldose reductase immunoreactivity, was detected in rat islet plasma membrane fractions purified by a polyethylene glycol-Dextran gradient or by a sucrose gradient. This is interesting in view of the fact that voltage-gated potassium channel beta subunits that contain aldehyde and aldose reductase-like NADPH-binding motifs have been detected in plasma membrane fractions of islets [Receptors and Channels 7: 237-243, 2000] and suggests that NADPH might have a yet unknown function in regulating activity of these potassium channels. Reductases may be present in cytosol to protect the insulin cell from molecules that cause oxidative injury.