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

Recent evidence of a pyruvate malate shuttle capable of transporting a larg e amount of NADPH equivalents out of mitochondria in pancreatic islets sugg ests that cytosolic NADP(H) plays a role in beta cell metabolism. To obtain clues about these processes the activities of several NADPH-utilizing enzy mes were estimated in pancreatic islets. Low levels of pyrroquinolone quino ne (PQQ) and low levels of enzyme activity that reduce PQQ were found in is lets. Low activities of palmitoyl-CoA and stearoyl-CoA desaturases were als o detected. Significant activities of glutathione reductase, aldose reducta se (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 glu cose-induced insulin release nor glucose metabolism in islets indicating th at these reductases are not directly involved in glucose-induced insulin re action. Over 90% of aldose reductase plus aldehyde reductase enzyme activit y 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 activ ity, as well as aldose reductase immunoreactivity, was detected in rat isle t plasma membrane fractions purified by a polyethylene glycol-Dextran gradi ent or by a sucrose gradient. This is interesting in view of the fact that voltage-gated potassium channel beta subunits that contain aldehyde and ald ose reductase-like NADPH-binding motifs have been detected in plasma membra ne fractions of islets [Receptors and Channels 7: 237-243, 2000] and sugges ts that NADPH might have a yet unknown function in regulating activity of t hese potassium channels. Reductases may be present in cytosol to protect th e insulin cell from molecules that cause oxidative injury.