A Study of Drug Metabolism Linked to Cytochrome P‐450 in Isolated Rat‐Liver Cells

Isolated rat liver cells were found to catalyze the cytochrome P‐450 dependent, oxidative metabolism of alprenolol [1‐(2‐allylphenoxy)‐3‐isopropylaminopropanol] at a rate similar to that obtained with the isolated microsomal fraction in the presence of a NADPH‐generating system. Alprenolol was rapidly taken up into the liver cells and the apparent Michaelis constant was similar to that obtained with liver microsomes. Further, inhibitors of drug metabolism such as SKF 525‐A and metyrapone showed similar inhibitory patterns in the cellular and microsomal systems. With liver cells isolated from control rats, NADPH generation from endogenous substrates was sufficient to support optimal alprenolol metabolism, whereas the addition of glucose or lactate stimulated this rate in liver cells isolated from starved, phenobarbital‐treated rats. Inhibitors of mitochondrial respiration such as rotenone, antimycin A and KCN and the uncoupler carbonyl cyanide‐p‐trifluoromethoxy‐phenyl hydrazone inhibited cellular alprenolol metabolism, presumably by lowering the ATP/ADP ratio thereby interfering with NADPH generation via the malic enzyme system; this effect was more pronounced in liver cells from starved rats. Furthermore, in analogy with previous findings, the rate of gluconeogenesis from lactate was inhibited during alprenolol metabolism; this effect in turn was most pronounced in liver cells from phenobarbital‐treated rats. The findings indicate that under certain conditions, the level of cytoplasmic NADPH may become rate‐limiting for drug metabolism linked to cytochrome P ‐450 and support the previous assumption that in fed rats, generation of cytoplasmic NADPH is mainly via the pentose phosphate shunt whereas the malic enzyme system becomes more important for this purpose during starvation.