EPR spectroscopy studies on the structural transition of nitrosyl hemoglobin in the arterial-venous cycle of DEANO-treated rats as it relates to the proposed nitrosyl hemoglobin/nitrosothiol hemoglobin exchange.

In vivo studies show a dynamic cycle in which alpha-nitrosylated hemoglobin is mainly in the relaxed state in arterial blood of rats treated with 2-(N,N-diethylamino)-diazenolate-2-oxide, but converts mainly to the tense state during the arterial-venous transit. A detailed analysis shows that different electron paramagnetic resonance spectra recorded for alpha-nitrosyl hemoglobin in arterial and venous blood at 77 K originate only from a different ratio between 5- and 6-coordinate heme without any change in the concentration of nitrosyl hemoglobin. In venous blood, the five- and six-coordination equilibrium of the alpha-nitrosyl heme is shifted in favor of the 5-coordinate state (58% venous vs. 20% arterial). These results are not consistent with the recently proposed exchange of nitrosyl heme with the beta-93 nitrosothiol group of hemoglobin during the arterial-venous cycle.