Tyrosine, Nitration, and Nitrosation

So what can be learned from studies of phenol/ONOO reactions which can benefit the search for and detection of specific ONOO footprints in vivo By far the most abundant phenolic compound in cells and tissues is tyrosine, primarily as residues in proteins. Tyrosine residues are readily nitrated by ONOO without the need for transition metals. Transition metals such as Fe(III) or Cu(II) catalyze nitration, and the nitration yield increases linearly with metal concentration (Beckman eta/., 1992). However, significant increases in nitration yield occur only at transition metal concentrations that exceed those likely to ever be present in vivo. 

Physiological concentrations of Cu/Zn SOD catalyze nitration (Ischiro-poulos etal., 1992b) and enhance nitration yield, whereas similar concentrations of either free or chelated copper do not. These findings indicate that SOD it is not simply acting as an endogenous copper chelator. ONOO bears a structural resemblance to superoxide and may be directed to the 

It can be argued that nitrotyrosine may not be a specific marker for ONOO , because -N02 has also been shown to nitrate phenolics such as tyrosine in vitro (Prutz et al., 1985). However, it is important to consider three points (1) the likelihood of -NOi formation in vivo, (2) the maximum amount that could be formed even under ideal conditions, and (3) the nitration efficiency by NO2. Nitration of tyrosine is a two-electron oxidation reaction N02 is a one-electron oxidant and, therefore, quite inefficient at nitrating. More important to this argument is the slow rate of -N02 formation from in vivo concentrations of nitric oxide, as well as the maximum amount that could possibly be formed (see earlier discussion). Even when 

2 mM nitric oxide was added to a phenol solution at ambient oxygen, a condition that results in rapid and substantial -N02 formation and nitrosa-tion of phenol, no nitrated products were seen (Figs. 4 and 5). 

Aerobic additions of nitric oxide, ONOO , or both all resulted in formation of only 4-nitrosophenol, whereas both 2- and 4-nitrophenol were formed from ONOO . The lack of 2-nitrosophenol production has particular relevance to the tyrosine products, which could be produced and measured either in vivo or in vitro. The side chain of tyrosine exists at the position that corresponds to the 4-position of phenol. Thus, 4- or p-nitrosotyrosine cannot exist. The o-position to the hydroxyl of tyrosine is equivalent to the 2-position of phenol. 

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