Disruption of pyridine nucleotide redox status during oxidative challenge at normal and low-glucose states: implications for cellular adenosine triphosphate, mitochondrial respiratory activity, and reducing capacity in colon epithelial cells.

We recently demonstrated that menadione (MQ), a redox cycling quinone, mediated the loss of mitochondrial glutathione/glutathione disulfide redox balance. In this study, we showed that MQ significantly disrupted cellular pyridine nucleotide (NAD(+)/NADH, NADP(+)/NADPH) redox balance that compromised cellular ATP, mitochondrial respiratory activity, and NADPH-dependent reducing capacity in colonic epithelial cells, a scenario that was exaggerated by low glucose. In the cytosol, MQ induced NAD(+) loss concurrent with increased NADP(+) and NAD kinase activity, but decreased NADPH. In the mitochondria, NADH loss occurred in conjunction with increased nicotinamide nucleotide transhydrogenase activity and NADP(+), and decreased NADPH. These results are consistent with cytosolic NAD(+)-to-NADP(+) and mitochondrial NADH-to-NADPH shifts, but compromised NADPH availability. Thus, despite the sacrifice of NAD(+)/NADH in favor of NADPH generation, steady-state NADPH levels were not maintained during MQ challenge. Impairments of cellular bioenergetics were evidenced by ATP losses and increased mitochondrial O(2) dependence of pyridine nucleotide oxidation-reduction; half-maximal oxidation (P(50)) was 10-fold higher in low glucose, which was lowered by glutamate or succinate supplementation. This exaggerated O(2) dependence is consistent with increased O(2) diversion to nonmitochondrial O(2) consumption by MQ-semiquinone redox cycling secondary to decreased NADPH-dependent MQ detoxication at low glucose, a situation that was corrected by glucose-sparing mitochondrial substrates.