Inhibition of the yeast metal reductase heme protein fre1 by nitric oxide (NO): a model for inhibition of NADPH oxidase by NO

Nitric oxide (NO) has been found to inhibit the actions of the transmembrane metal reductase Fre1 in the yeast Saccharomyces cerevisiae. This membrane-spanning heme protein is homologous to the gp91PHOX protein of the NADPH oxidase enzyme complex and is responsible for reducing extracellular oxidized metals (i.e., ferric and cupric ions) before high-affinity uptake. Consistent with its role in metal metabolism, inhibition of Fre1 by NO also inhibited yeast growth in low-iron medium. Inhibition by NO was found to be O2-dependent and irreversible. Further examination of the chemistry responsible for activity loss shows that the generation of N2O3 via NO–O2 chemistry was responsible for the activity loss, possibly via nitrosation of the protein followed by loss of the heme prosthetic group.