HbSNO

Perhaps the most physiologically important metalloprotein to be S-nitrosated by NO is hemoglobin (Hb). The S-nitrosation of Hb-Cys 93 by NO was first demonstrated by Stamler and coworkers in 1996 (Jia et al., 1996 Stamler et al., 1997). To this day HbSNO remains controversial with respect to its mechanism of formation and physiological relevance. 

The wealth of data on HbSNO formation has given rise to two broad interpretations that either support the hypothesis that the S-nitrosation of Hb-Cys 93 is redox catalyzed by the heme and is under allosteric control or that it is N2 03 or N02 -mediated without the involvement of the hemes. 

The main point of argument with the formation of HbSNO is that in vitro the exposure of NO to oxyhemoglobin (HbFe(II)-02) results in the production of methe-moglobin (HbFe(III)) plus nitrate (Eq. (2)). 

Since the rate constant for this reaction is estimated to be 9xl07M 1s 1 (Eich et al, 1996 Herold et al, 2001) the contention is that this reaction would predominate in vivo, thus minimizing the formation of HbSNO. 

In the following section the evidence for and against a role of the heme in HbSNO formation is discussed. 

The proof of this hypothesis was that CN, which binds tightly to MetHb, resulted in no detectable nitrosylHb. The fact that HbSNO was still formed even with the CN -blocked Hb suggested that HbSNO could be formed in a heme-independent manner. In RBC suspensions, under the same experimental conditions, a -2-fold larger amount of nitrosylHb was detected in comparison to free oxyHb but no HbSNO was detected. Again the nitrosylHb-formation was attenuated with CN . 

In early 2003 Stamler and Singel and coworkers (Luchsinger et al., 2003) proposed a MetHb intermediate route for HbSNO formation. This mechanism was very similar to that proposed by Han et al. (2002). In this study, they presented EPR evidence that... 

The oft-touted argument against a physiological role for HbSNO is that even if it does form in RBCs, the Hb-bound NO has to make quite a journey to get to smooth muscle cells it must first cross the RBC plasma membrane, then the RBC free zone (Liao et al., 1999), then cross the endothelial cell membranes twice and finally go through the smooth muscle membrane. Why would nature adopt such a convoluted route when the endothelial cells next to the smooth muscle cells are producing membrane-diffusable NO ... 

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