Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produ ces low-spin ferric heme hemichromes which have been characterized by elect ron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2 .21, and 2.97, Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59, The formation of thes e species goes undetected by UV-vis spectroscopy, but the oxygenation of my oglobin to metmyoglobin is correlated with complete conversion of nitric ox ide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.