Autoinhibition of endothelial nitric-oxide synthase. Identification of an electron transfer control element.

The primary sequences of the three mammalian nitric- oxide synthase (NOS) isoforms differ by the insertion of a 52-55-amino acid loop into the reductase domains of the endothelial (eNOS) and neuronal (nNOS), but not inducible (iNOS). On the basis of studies of peptide derivatives as inhibitors of.NO formation and calmodulin (CaM) binding (Salerno, J. C., Harris, D. E., Irizarry, K., Patel, B., Morales, A. J., Smith, S. M., Martasek, P., Roman, L. J., Masters, B. S., Jones, C. L., Weissman, B. A., Lane, P., Liu, Q., and Gross, S. S. (1997) J. Biol. Chem. 272, 29769-29777), the insert has been proposed to be an autoinhibitory element. We have examined the role of the insert in its native protein context by deleting the insert from both wild-type eNOS and from chimeras obtained by swapping the reductase domains of the three NOS isoforms. The Ca2+ concentrations required to activate the enzymes decrease significantly when the insert is deleted, consistent with suppression of autoinhibition. Furthermore, removal of the insert greatly enhances the maximal activity of wild-type eNOS, the least active of the three isoforms. Despite the correlation between reductase and overall enzymatic activity for the wild-type and chimeric NOS proteins, the loop-free eNOS still requires CaM to synthesize.NO. However, the reductive activity of the CaM-free, loop-deleted eNOS is enhanced significantly over that of CaM-free wild-type eNOS and approaches the same level as that of CaM-bound wild-type eNOS. Thus, the inhibitory effect of the loop on both the eNOS reductase and. NO-synthesizing activities may have an origin distinct from the loop's inhibitory effects on the binding of CaM and the concomitant activation of the reductase and.NO-synthesizing activities. The eNOS insert not only inhibits activation of the enzyme by CaM but also contributes to the relatively low overall activity of this NOS isoform.