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Fig. 1. Molecular structures of CysNO, GSNO and SNAP.

CysNO

Fig. 1. Molecular structures of CysNO, GSNO and SNAP.

(ESH) and ovothiols [5]. ESH is a low-molecular-weight thiol with established reactivity towards GSNO [6], capacity to scavenge singlet oxygen and hydroxyl radicals, and is found in millimolar concentrations in speci c tissues like liver, kidney and erythrocytes [7]. ESH was not reported to form S-nitrosothiols, but it may form disul des with cysteinic thiols. Although the reactivity towards GSNO is intriguing, the biological function of such histidine-based thiols is still unclear. Therefore, such unconventional thiols will not be considered further in this chapter.

Finally, it should be mentioned that S-nitrosation is only one of many pathways via which nitroso moieties can be incorporated into proteins. Organic molecules and peptides may undergo N-, O-, C- as well as S-nitrosation, depending on the structural motif to which the NO moiety is attached. Mixed nitrosations are of course possible also. An interesting example is albumin which may be N-nitrosated on one of its two tryptophan residues as well as S-nitrosated on the Cys34 residue [8]. Signi cantly, the N-nitrosated tryptophan residue also elicited vasodilatory response from precontracted aortic rings of rabbits [8]. Since the nitrosation was carried out under extreme non-physiological conditions (acidi cation of nitrite), it remains unclear whether this N-nitrosation of tryptophans has signi cance for in vivo conditions.

In contrast, the physiological signi cance of S-nitrosation has been proven beyond doubt. Many examples will be given at the end of this chapter. Under biological conditions, S-nitrosation is facile, fast and affects a wide range of proteins in vitro and in vivo. Judging from the citation numbers in scienti c literature, the nitrosation of sulfhydryl groups has highest relevance for physiology so far. Therefore, this chapter will be primarily concerned with S-nitrosocompounds.

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