NAD(P)H is known to be oxidized by singlet molecular oxygen, perhydroxyl radical, and hydroxyl radical. In marked contrast to these reactive oxygen species, NAD(P)H is stable in the presence of micromolar concentrations of H2O2. The experiments herein demonstrate that NADPH is rapidly oxidized by H2O2 in the presence of a heme-peptide. The oxidation product is enzymatically active NADP+. In the absence of NADPH, the heme-peptide undergoes rapid degradation via reaction with H2O2. In the presence of NADPH, the reduced nucleotide is oxidized to NADP and the heme-peptide is partially protected from oxidation. It is suggested that under certain conditions the reduced nucleotides may contribute to the protection of intracellular heme moieties from degradation engendered by endogenous or exogenous H2O2.