Reduced nicotinamide adenine dinucleotide phosphate oxidase in adipocyte plasma membrane and its activation by insulin: Possible role in the hormone's effects on adenylate cyclase and the hexose monophosphate shunt

Abstract

An oxidase activity utilizing reduced nicotinamide adenine dinucleotide phosphate (NADPH) and producing H 2O 2 was observed in intact adipocytes of rat, as well as in the isolated plasma membranes of these cells. A stoichiometry of 1 mol of H 2O 2 production per mole of NADPH disappearance was found with isolated plasma membranes. Activation of this enzyme (R) was produced by pretreatment of cells with insulin, dithiothreitol, or sulfhydryl inhibitors, e.g., p-chloromercuribenzoate or tosyl- l-lysine chloromethyl ketone. All of these agents also stimulated glucose oxidation via the hexose monophosphate shunt. Activation of R was also observed with biologically active derivatives of insulin, e.g., proinsulin or desalanine insulin, but not with an inactive derivative, desoctapeptide insulin. The enzyme could not be activated by exposing the cells to membrane perturbants, e.g., hypotonic conditions or Triton X-100 (0.01–0.1%). The enzyme activity in the plasma membrane had a pH optimum at 6.0 and, from the Lineweaver-Burke plot, V was determined at 230 nmol and K m for NADPH was at 5.8 × 10 −5, m. The activity remained unaltered in the presence of sodium azide or cyanide. Preincubation of adipocytes with insulin or SH reagents or direct addition of oxidants, e.g., H 2O 2, potassium ferricyanide, or phenazine methosulfate, to the membranes also caused inhibition of adenylate cyclase (AC). This enzyme activity could be restored in these preparations by adding thiols. It is suggested that inhibition of AC in whole cells in response to insulin may be caused by oxidation of its SH groups by the H 2O 2 generated from the activated NADPH oxidase. Reversal of this inhibition may involve cellular reducing equivalents. The evidence suggests that the plasma membrane enzymes, i.e., NADPH oxidase and adenylate cyclase, are controlled, in part, by the intracellular redox potential.