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.
Footnotes
- Received February 16, 1998.
- Accepted May 1, 1998.
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Send reprint requests to: Dr. Paul R. Ortiz de Montellano, Department of Pharmaceutical Chemistry, School of Pharmacy, S-926, University of California, San Francisco, CA 94143-0446. E-mail:ortiz{at}cgl.ucsf.edu
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This work was supported by National Institutes of Health grant GM25515.
- The American Society for Pharmacology and Experimental Therapeutics
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