Effects of Nitric oxide on Cytochrome P450-mediated Drug Metabolism
Various studies have shown that the ability of the liver to metabolize drugs is compromised in patients with infectious or inflammatory diseases. Large quantities of nitric oxide (NO) released during inflammation are implicated in these observations. However, most of these studies concentrated on the relatively long-term (6-24 hr) effects of NO at the transcriptional level. Because NO reacts strongly with hemoproteins and its concentrations are upregulated shortly after inflammatory processes, we hypothesized that NO affects P450 activity rapidly and directly. This hypothesis was based on a preliminary study, in which we showed that perfusion of isolated rat livers with endotoxin-contaminated albumin for 3 h resulted in a significant decrease in P450 activities in the absence of any change in the isoenzyme protein levels. The decrease in enzyme activity was associated with a substantial increase in the perfusate NO levels (Chapter 2). In a subsequent study, we showed that the perfusion of isolated rat livers for 1 hr with NO donors substantially decreased the P450 activities directly, rapidly, concentration-dependently, and enzyme-selectively (Chapter 3). A further study examined the time course and degree of reversibility of NO effects on P450 activities. NO exposure caused an enzyme-selective and time-dependent decline in P450 activities with both reversible and irreversible mechanisms (Chapter 4). Next, the direct effects of NO donors on the kinetic parameters of various P450 enzymes were investigated. Results from this study indicated that NO can potentially alter both the Vmax and Km of various substrates selectively (Chapter 5). Finally, the effects of NO on the metabolism of chlorzoxazone (CYP2E1) and dextromethorphan (CYP2D1) were examined in the intact IPRL, as opposed to our previous studies in microsomes. These two substrates were selected as model drugs because our microsomal studies had indicated that CYP2E1 and 2D1 activities were most and least affected by NO, respectively. Treatment with NO donors resulted in a significant decline in the concentrations of the chlorzoxazone metabolite in the outlet perfusate. Conversely, NO did not affect the metabolism of dextromethorphan in intact livers (Chapter 6). In conclusion, our studies indicate that NO causes a rapid, direct, concentration-dependent, and enzyme-selective decline in the activities of cytochromes P450 in an IPRL model.