Microsomal target proteins of metabolically activated aromatic hydrocarbons

The specificity of binding to microsomal proteins of metabolically activated hydrocarbons has been studied. Radioactively labelled benzene, phenol, chlorobenzene, BP and MC were incubated with liver microsomes from control, phenobarbital- and MC-treated rats in the presence of an NADPH-generating system. The patterns of metabolite binding to microsomal proteins were examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and fluorography. Benzene, phenol and chlorobenzene metabolites showed one type of binding pattern dominated by a band at 72 000 Mr. This band was strong both in control and induced microsomes. Additional radioactive bands were seen in the 50 000--60 000 Mr region particularly in MC-induced microsomes. BP and MC metabolites showed a different type of binding pattern with incorporation of radioactivity into several fractions in the 50 000--60 000 Mr region of MC-induced microsomes. Two other strongly labelled fractions occurred at 68 000 and 72 000 Mr. The incorporation was low into control and phenobarbital-induced microsomes. Two labelled bands (Mr 56 000 and 72 000) were common for all hydrocarbons in MC-induced microsomes. The 56 000 Mr band had the same mobility in the gel as an MC-induced protein likely to be cytochrome P-448. The NADPH-generating system was essential for metabolite binding and GSH and UDPGA greatly reduced binding. We suggest that differences in metabolite binding patterns reflect differences in the routes of metabolite formation and that activated hydrocarbons are likely to bind to proteins close to their site of formation.