Endogenous ROS

Pyruvate Dehydrogenase Complex

Pyruvate Dehydrogenase Complex generates significant amount of reactive oxygen when supply of NAD+ is not sufficient and when the ratio dihydrolipoate/lipoate is increased:

Experiments conducted using isolated enzyme have demonstrated PDC-stimulated rates of O2•− to be maximal under conditions in which the steady-state ratio of dihydrolipoate to lipoate within E2 is shifted in favor of the reduced form and supply of NAD+ is decreased (i.e., ↑dihydrolipoate/lipoate, ↑NADH/NAD+).

Under these conditions, it is believed that E3 catalyzes a 1e− oxidation reaction between O2 and E2-bound dihydrolipoate, in turn leading to either O2•− and/or a thiyl radical. …. Specifically, pyruvate-supported H2O2 production by PDC was found to be remarkably sensitive to mitochondrial glutathione depletion, generating rates of H2O2 emission far greater than any other site/substrate combination tested.

Oxidation of the cellular glutathione pool in response to high fat feeding was subsequently shown to elevate pyruvate-supported H2O2 emission within skeletal muscle, as well as increase the sensitivity of PDC to glutathione depletion.

To our knowledge this is the first evidence that PDC is capable of generating substantial rates of H2O2 production and thus could potentially represent a major source for mitochondrial derived H2O2 within skeletal muscle, especially in response to nutrient overload or other conditions in which the mitochondrial glutathione pool has undergone a shift to a more oxidized state (e.g., metabolic disease). (R1)

Xanthine Oxidase

In this study we found that H2O2-dependent calcium release from the ER caused conversion of XDH to XO, and that this was prevented by nitric oxide. This may represent a feed-forward mechanism in many disease processes whereby H2O2 can stimulate further ROS production through XDH-to-XO conversion. (R3)

Highlights:

1. Nitric Oxide prevents conversion of XDH to ROS-generating XO.
2. Molybdenum Cofactor supply supports ROS generation by Xanthine Oxidase.
3. Hydrogen Peroxide stimulates conversion of XDH to XO and by doing so increases ROS production.

NADPH Oxidase

Recent studies have revealed that NADPH oxidase, a membrane-bound enzyme complex that catalyzes the production of a superoxide free radical, is one of the major sources of cellular reactive oxygen species in acute neurological disorders. … While NADPH oxidase (NOX) contributes to normal physiological processes in the central nervous system, excessive activation of NOX results in neuronal damage in the setting of acute brain injury. (R2)