Mechanism of nitrite-dependent NO synthesis by human sulfite oxidase

In addition to nitric oxide (NO) synthases, molybdenum-dependent enzymes have been reported to reduce nitrite to produce NO. Here, we report the stoichiometric reduction of nitrite to NO by human sulfite oxidase, a mitochondrial intermembrane space enzyme primarily involved in cysteine catabolism. Kinetic and spectroscopic studies provide evidence for direct nitrite coordination at the molybdenum center followed by an inner shell electron transfer mechanism. In the presence of the physiological electron acceptor cytochrome c , we were able to close the catalytic cycle of sulfite-dependent nitrite reduction thus leading to steady state NO synthesis, a finding that strongly supports a physiological relevance of sulfite oxidase-dependent NO formation. By engineering sulfite oxidase variants with reduced intra-molecular electron transfer rate, we were able to increase NO generation efficacy by one order of magnitude, providing a mechanistic tool to tune NO synthesis by sulfite oxidase.