Abstract

The neuronal nitric oxide synthase isoform nNOSμ, which is expressed in striated muscle, differs from nNOSα, the major brain isoform, by the insertion of 34 amino acid residues between the calmodulin- and flavin-binding domains [J Biol Chem 271:11204–11208 (1996)]. We show here that recombinant, purified nNOSμ, despite the peptide insertion, has the same spectroscopic properties, l-argininek _cat andK _m values, optimal pH, and calmodulin binding affinity constant as nNOSα. However, nNOSμ consumes NADPH and reduces cytochrome c at approximately half the rate of nNOSα. The rates of degradation of the two proteins by rat brain and muscle homogenates show that nNOSμ is degraded more slowly than nNOSα. The in vitro half-lives of nNOSα and nNOSμ are 12 and 50 min, respectively, and calpain is important for this degradation. These short in vitro half-lives suggest that the nNOS isoforms are susceptible to rapid degradation in vivo. The elevated (20-fold) levels of calpain in diseased muscle tissue in Duchenne muscular dystrophy, and the hydrolytic sensitivity of both nNOSμ and nNOSα to this enzyme, may contribute to the deficiency of nNOS activity in the diseased tissue.