Vascular Effects of NO

Abstract

Nitric oxide synthase (NOS) III is a constitutively expressed 135-kDa protein predominantly associated with the particulate subcellular fraction, suggesting that the native enzyme is a membrane-bound protein [1-4]. Membrane association appears to be achieved by attachment of myristic acid to the amino terminal end of the enzyme and, as such, is consistent with reports that NOS cDNA contains a consensus sequence for cotranslational modification of the enzyme by N-terminal myristoylation [5-9]. Prevention of myristic acid incorporation in site-directed mutagenesis experiments converts the membrane-associated NOS to a cytosolic form [310]; however, this intervention has been reported to have little effect on enzyme activity [10]. Myristoylation alone, however, provides barely enough energy to anchor a protein to a lipid bilayer; therefore, it seems likely that other factors are important in determining the fraction of the enzyme associated with the plasma membrane (e.g., additional hydrophobic interactions owing to reversible palmitoylation of a nearby cysteine residue). In cases of such a dual protein acylation, the combined hydrophobic interactions of the two covalently attached lipid moieties anchor the protein firmly to the plasma membrane and its attachment could be regulated by palmitate turnover. Indeed, the membrane-bound NO synthase is palmitoylated, and depalmitoylation of the endothelial NO synthase, along with its translocation to the cytosol, has been reported following exposure to the receptor-dependent agonist bradykinin [11]. Because agonist-induced depalmitoylation was reportedly maximal only after 15-30 min, this phenomenon would appear to be associated with inactivation rather than activation of NOS activity. The role of actively regulated NOSIIIpalmitoylation in determining its cellular localization remains controversial [12].