Pyridine nucleotide hydrolysis and interconversion in rat hepatocytes during oxidative stress

A characteristic feature of many types of chemically induced oxidative stress is a depletion of the pyridine nucleotide NAD+. This has been attributed to either its hydrolysis to nicotinamide and ADP-ribose or to its phosphorylation (interconversion) to NADP+. In this study the exposure of rat hepatocytes to either tert-butyl hydroperoxide (250-750 microM) or 2,3-dimethoxy-1,4-naphthoquinone (50 microM) resulted in a rapid depletion of NAD+ with no change in the level of NAD. The depletion of NAD+ was accompanied by an increase in nicotinamide. The rate of NAD+ deletion induced by tert-butyl hydroperoxide (500 microM) and 2,3-dimethoxy-1,4-naphthoquinone (50 microM) was reduced by preincubating the hepatocytes for 1 hr with either 3-aminobenzamide (20 mM), nicotinamide (10 mM) or theophylline (7.5 mM), potent inhibitors of poly(ADP-ribose)polymerase. In cells exposed to 2,3-dimethoxy-1,4-naphthoquinone (50 microM) extensive oxidation of NADPH to NADP+ was observed; this was followed by an increase in the level of NADP(+) + NADPH (NADP(H)). However, no change in the total pyridine nucleotide (NAD(H) + NADP(H)) pool was detected. Exposure to tert-butyl hydroperoxide resulted in the oxidation of NADPH to NADP+ and a decrease in total pyridine nucleotide pool. These results suggest that during oxidative stress, NAD+ is hydrolysed to nicotinamide, possibly by the activation of poly(ADP-ribose)polymerase and that the depletion of NAD+ is independent of the increase in NADP+. Furthermore, no evidence of an interconversion of NAD+ to NADP+ was found.