Putative “protein nitratases,” which catalyze denitration of peroxynitrite (PN)‐treated proteins, were detected in the crude extract of dog prostate. Nitratase activity was monitored by the decreased intensity of nitrotyrosine immunoreactive‐bands in Western blot and increased nitrate level in dialysate of incubation mixture, which contained prostate crude extract, protease inhibitors and a PN‐treated substrate, such as treated histone (III‐S), BSA, invertase, or polylysine. Nitratases were activated by preincubation with m‐calpain/Ca2+. Furthermore, after denitration, the activity of PN/DTT‐treated invertase decreased to the similar activity level of DTT‐treated invertase. At least two different types of nitratases may occur: type I, reductant‐dependent, and type II, reductant‐independent.

Putative 'protein nitratases,' which catalyze denitration of peroxynitrite (PN)-treated proteins, were detected in the homogenate/crude extract of rat brains and hearts. Nitratase activity was monitored by the decreased intensity of nitrotyrosine immunoreactive-bands in Western blot and increased nitrate level in dialysate of incubation mixture, which contained homogenate/crude extract, protease inhibitors and a PN-treated substrate, such as treated histone (III-S), BSA or invertase. Enhanced activity of nitratases was noted by preincubating crude extract with Ca2+. In addition, at least two types of nitratases may occur: type I, reductant-dependent, and type II, reductant- independent. Furthermore, upon denitration, the activity of PN-treated invertase increased to the same activity level of the untreated invertase. The overall reaction catalyzed by nitratases for denitration of nitrotyrosine residues in protein could be as follows: Protein-Tyr-NO2 + H2O --> Protein-Tyr-H + H+ + NO3-. The nitration/denitration of protein-tyrosine may be crucial in regulating signal transduction.

Unlike the formation of nitrosothiols by nitrous acid, our study revealed that NO2- effectively reacted with L-cysteine or reduced glutathione (GSH) at pH 7.0 and 7.4, to form orange-pink products of S-nitrosocysteine (CySNO) or S-nitrosoglutathione (GSNO). The reactions were in a concentration-dependent manner. These products exhibited not only peak absorbances at around 340 and 540 nm, but also unique colors and patterns of mobility on cellulose thin layer chromatographic plates. In comparison, the S-nitrosation of dithiothreitol was noted exclusively under acidic pH. In addition, the S-nitrosation of hemoglobin (Hb) by either peroxynitrite (PN) or NO2- at pH 6.0 was detected via Western blot. The half-life of degradation of CySNO in NO2- solution was significantly shorter than that of GSNO at a wide range of pH. In the absence of NO2-, degradation of GSNO was facilitated by incubation with L-cysteine, but not L-serine. In the signaling process involving NO -->PN --> NO2- --> CySNO/GSNO --> NO, L-cysteine may function as a NO-carrier to reach shorter-distance targets, and also an "activator" to release NO from GSNO. Furthermore, L-cysteine may play a vital role in reducing (severe) oxidative stress.

After removing nonspecific immunoreactivities from crude extract by immunoaffinity chromatography, an immunoreactive-band at 60kDa of constitutive nitric oxide synthase (cNOS) from Saccharomyces cerevisiae was detected by Western blot using mouse monoclonal anti-neuronal NOS (cNOS). The activity of yeast cNOS, which was prepared by either histone-agarose chromatography or anti-neuronal NOS immunoprecipitation, was monitored by the formation of citrulline. Yeast cNOS was activated in the presence of calmodulin and arginine, whereas it was inhibited by L-NAME, a mammalian NOS inhibitor. Moreover, actinomycin-D decreased the extracellular and the intracellular levels of nitrate and nitrite which had been converted from NO. The results suggest that cNOS occurs in unicellular eukaryotes and the enzyme activity can be regulated.

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