KINETIC CHARACTERISTICS OF NITRIC-OXIDE SYNTHASE FROM RAT-BRAIN

The relationship between the rate of synthesis of nitric oxide (NO) and guanylate cyclase stimulation was used to characterize the kinetics of the NO synthase from rat forebrain and of some inhibitors of this enzyme. The NO synthase had an absolute requirement for L-arginine and NADPH and did not require any other cofactors. The enzyme had a V(max.) of 42 pmol of NO formed·min-1·mg of protein-1 and a K(m) for L-arginine of 8.4 μM. Three analogues of L-arginine, namely N(G)-monomethyl-L-arginine, N(G)-nitro-L-arginine and N(G)-iminoethyl-L-ornithine inhibited the brain NO synthase. All three compounds were competitive inhibitors of the enzyme with K(i) values of 0.7, 0.4 and 1.2 μM respectively.