NITRIC-OXIDE SYNTHASE IN CAT BRAIN - COFACTORS-ENZYME-SUBSTRATE INTERACTION

Nitric oxide, derived from L-arginine by the enzyme nitric oxide synth ase, is an activator of the soluble guanylate cyclase and a cellular m essenger. This work demonstrates that, in cat brain, the neuronal cons titutive nitric oxide synthase activity is a) NADPH/calcium dependent, b) independent upon exogenous calmodulin in crude brain supernatant, c) significantly enhanced by exogenous FAD and tetrahydrobiopterin (V- max: 118 instead of 59.4 pmol of citrulline formed . mg of prot .(-1) min(-1), d) inhibited by calcium chelators and calmodulin antagonist, and e) present in several neuroanatomical structures. Moreover, the K- m value for L-arginine was of 11 mu M instead of 41 mu M in the presen ce of FAD and tetrahydrobiopterin in the incubation mixture, thus demo nstrating that these cofactors are able to stabilize the enzyme-substr ate interactions.