Nitric oxide synthases: structure, function and inhibition

This review concentrates on advances in nitric oxide synthase (NOS) structu re, function and inhibition made in the last seven years, during which time substantial advances have been made in our understanding of this enzyme fa mily. There is now information on the enzyme structure at all levels from p rimary (amino acid sequence) to quaternary (dimerization, association with other proteins) structure. The crystal structures of the oxygenase domains of inducible NOS (iNOS) and vascular endothelial NOS (eNOS) allow us to int erpret other information in the context of this important part of the enzym e, with its binding sites for iron protoporphyrin IX (haem), biopterin, L-a rginine, and the many inhibitors which interact with them. The exact nature of the NOS reaction, its mechanism and its products continue to be sources of controversy. The role of the biopterin cofactor is now becoming clearer , with emerging data implicating one-electron redox cycling as well as the multiple allosteric effects on enzyme activity. Regulation of the NOSs has been described at all levels from gene transcription to covalent modificati on and allosteric regulation of the enzyme itself. A wide range of NOS inhi bitors have been discussed, interacting with the enzyme in diverse ways in terms of site and mechanism of inhibition, time-dependence and selectivity for individual isoforms, although there are many pitfalls and misunderstand ings of these aspects. Highly selective inhibitors of iNOS versus eNOS and neuronal NOS have been identified and some of these have potential in the t reatment of a range of inflammatory and other conditions in which iNOS has been implicated.