We have previously reported various inductive effects of nitric oxide on human PBMC. We describe a novel and potentially important mechanism of nitric oxide signaling-through direct activation of guanine nucleotide-binding proteins (G proteins). We have found that nitric oxide treatment of membranes isolated from fresh human PBMC enhances the ability of these membranes to hydrolyze [gamma-32P]GTP and bind [gamma-35S]GTP. In addition, treatment of whole cells with nitric oxide yielded membranes with enhanced GTPase activity. Furthermore, the GTPase activity of pure, recombinant Gs alpha, Gi alpha 1, and p21ras was greatly enhanced by nitric oxide. In support of the existence of this pathway in whole cells, we found that the G protein inhibitor, GDP-beta-S, blocked NF-kappa B translocation induced by nitric oxide or LPS in permeabilized cells. In addition, nitric oxide greatly reduced the pertussis toxin-mediated ADP-ribosylation of 45- and 41-kDa proteins in membranes of these cells. Because G proteins play a central role in many diverse signaling systems, activation by an endogenous and inducible oxidant may represent a novel signaling pathway.