The stimulation of NADH oxidase activity of rat liver plasma membranes by guanine nucleotides may involve both guanine nucleotide-binding proteins of the plasma membrane and responses not mediated by classic heterotrimeric G proteins

Abstract

The stimulation of NADH oxidase activity of plasma membranes of rat liver observed with guanine nucleotides may involve both guanine nucleotide-binding proteins of the plasma membrane and responses not mediated by classic heterotrimeric G proteins. These conclusions are based on findings that detergent treatment and peptide antisera to a consensus guanine nucleotidebinding domain (GAGES) of Gα subunits of heterotrimeric G proteins reduced but did not eliminate the stimulation of NADH oxidase activity by guanine nucleotides. The proteins immunoprecipitated by the antisera, when added back to plasma membranes, stimulated the NADH oxidase activity. This stimulated rate was further stimulated by the addition of GTP but was not dependent upon guanine nucleotide presence. Additions of cytosol, either fractionated or unfractionated did not appear to stimulate the NADH oxidase activity of rat liver plasma membranes. The activities of the plasma membranes and the activities introduced by the cytosol fractions were nearly, but not entirely, additive. The results are suggestive of a subunit composition of the NADH oxidase but one distinct from that involving solely heterotrimeric G proteins. Also a strong dependence on cytosolic components, as found with the NADPH oxidase complex of neutrophils, is not obvious. In addition, the possibility that the NADH oxidase may exhibit an intrinsic re-sponse to guanine nucleotides, not dependent on accessory proteins, cannot be ruled out. Among the several bands immunoprecipitated with the antisera and reactive with the antisera on Western blots, were peptide bands in the molecular weight range ascribed to the NADH oxidase.