In vitro formation of S-nitrosohemoglobin in red cells by inducible nitric oxide synthase.

The present study demonstrates that NO produced in vitro by inducible nitric oxide synthase in red cells can convert hemoglobin contained in the red cells to S-nitrosohemoglobin. Experiments carried out either in the absence or in the presence of a low molecular weight thiol, such as cysteine, showed that in the first case the target of NO is heme-Fe2+. On the contrary, in the presence of cysteine, the first step is the formation of S-nitrosocysteine, followed by transfer of the NO group to a particular cysteine residue of beta-globin, cysteine 93. These results confirm previous data indicating the preferential formation of S-nitrosohemoglobin at that site by chemical methods [Ferranti et al. (1997) FEBS Lett. 400, 17-24], and the existence of a physiological mechanism of inactivation for NO circulating in blood. The analysis of S-nitrosohemoglobin can also allow the quantification of the NO levels in blood to be applied for in vitro and in vivo measurements.