Distribution of enzymes involved in metabolism of polycyclic aromatic hydrocarbons among rat liver endomembranes and plasma membranes.

The distribution of membrane-bound enzymes involved in the metabolism of polycyclic aromatic hydrocarbons was investigated in the same animals, in membrane fractions from rat liver: monooxygenase, cytochromes P-450 and P-420, NADPH-cytochrome P-450 reductase, cytochrome b5, NADH-cytochrome b5 reductase, epoxide hydrolase, glutathione-S-transferase, and UDP-glucuronyltransferase. The purity of the fractions (rough and smooth endoplasmic reticulum (ER), nuclei, nuclear membranes, Golgi apparatus, plasma membranes, and mitochondria) was controlled by electron microscopic morphometry, chemical analysis, and determination of marker enzyme activities. Specific activities invariably were highest in ER. With respect to the distribution between smooth and rough ER, the enzymes fell into two groups: Epoxide hydrolase, monooxygenase, NADPH-cytochrome P-450 reductase, NADH-cytochrome b5 reductase, and the concentrations of cytochromes P-450 and b5 were higher in smooth ER; by contrast, UDP-glucuronyltransferase (maximally activated) and membrane-associated glutathione-S-transferase activities were lower in smooth ER. The non-ER fractions also had significant activities of the enzymes, but to different extents: (1) Nuclei and nuclear membranes showed high UDP-glucuronyltransferase activities (on a phospholipid basis 55-65% of that of rough ER) which did not exhibit latency. (2) Plasma membrane, Golgi apparatus, and nuclear membrane fractions had relatively high epoxide hydrolase and NADH-cytochrome b5 reductase activities as well as high concentrations of cytochromes b5 and P-450 (20-30% of levels present in smooth ER on a phospholipid basis). (3) In mitochondria (whole organelles), glutathione-S-transferase. UDP-glucuronyltransferase, and NADH-cytochrome b5 reductase activities were high, in contrast to exceedingly low (or absent) amounts of cytochrome P-450 and activities of cytochrome P-450 reductase, monooxygenase, and epoxide hydrolase. The data show that (i) aromatic hydrocarbons are metabolized not only in ER membranes but also in other kinds of membranes, including Golgi apparatus and plasma membrane, and (ii) the mode and extent of this metabolism can differ in different membranes. Such differences of activities of enzymes present in the specific membranes might lead to differences of the intracellular distribution of reactive and inactive metabolites of aromatic hydrocarbons and thus may be important in diversification and regulation of metabolic pathways of carcinogenic compounds.