Recent studies on cytochrome P-450-linked functions in isolated rat liver cells

In rat liver cells isolated by perfusion in the perfusion in the presence of collagenase, the major portion of cytochrome P-450 is present in the oxidized, nonsubstrate-bound, low spin state. Drug addition to a suspension of liver cells results in the rapid formation of the cytochrome P-450 (Fe3+)-substrate complex which in turn is followed by the appearance of other species with different spectral characteristics before steady state drug monooxygenation is achieved. Cytochrome P-450-linked metabolism of various tested drugs and carcinogenic polycyclic hydrocarbons by isolated rat liver cells is as fast, or faster, as with rat liver microsomes supplemented with a NADPH generating system. Both experimental models respond similarily to phenobarbital or 3-methylcholanthrene pretreatment of the animals and to various of the wellknown inhibitors of drug metabolism. Except with liver cells isolated from fasted, phenobarbital-treated rats, generation of cytosolic NADPH seems sufficient to support optimal drug metabolism even in the absence of added substrates of intermediary metabolism. In isolated liver cells oxidized drug metabolites undergo subsequent metabolic conversion, most often to form the corresponding glucuronides and sulphates. These are readily excreted, whereas non-conjugated products, e.g. free phenols, tend to accumulate intracellularly. Cellular glucuronide formation is strongly inhibited by ethanol-presumably due to an unfavorable effect of the increased NADH/NAD+ ratio on the synthesis of uridine-5'-diphosphoglucuronic acid (UDPGA). In contrast, low concentrations of ethanol have no, or only a slight stimulatory effect on the cytochrome P-450-linked step of drug metabolism and there are indications that the oxidation of low concentrations of ethanol is in fact stimulated by a facilitated reoxidation of cytosolic NADH occuring during drug monooxygenation.