Possible role of RAC-GTPase-activating protein in the termination of superoxide production in phagocytic cells.

The mechanism leading to the termination of superoxide production of phagocytes is poorly understood. The aim of the present study was to investigate the involvement of the active (GTP-bound) form of the GTP-binding proteins in maintaining continuous electron transport through the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex. Activation of the enzyme was carried out under in vitro conditions and a shift from the active to the inactive form of the GTP-binding protein was attained (i) by addition of an excess of GDP to the assembled enzyme complex or (ii) by variation of the Rac-GTPase activating (Rac-GAP) capacity of the constituents of the cell-free system. Significant inhibition of O2*- production was observed when guanine dinucleotides were added after the assembly of the active enzyme complex. The effect was specific for GDP and GDP,S whereas ADP, CDP and UDP were ineffective. GTP was significantly less efficient in inducing superoxide production in a cell-free system containing endogenous GAP activity than in a system devoid of GAP activity. It is suggested that the active, GTP-bound form of Rac is required for sustained catalytic function and Rac-GAP proteins are involved in the downregulation of the oxidase.