Microsomal oxidation of bromo-, chloro- and fluorobiphenyls.

1. The metabolism of 2-, 3-, 4-bromo-, 2-, 4-chloro-, and 2-fluorobiphenyl by hepatic microsomes isolated from control and Aroclor 1254-treated rats and pigeons was studied. 2. Meta and para as well as dihydroxylated metabolites were detected, but para hydroxylation was the preferred route of metabolism with all of the substrates used. 3. The overall rates of hydroxylation were greater with hepatic microsomes from rats than from pigeons. 4. Treatment with Aroclor 1254, a potent inducer of hepatic monooxygenases, resulted in increased rates of metabolism and in the enhanced formation of diol metabolites. Metabolism of halobiphenyls by induced P450 isoenzymes altered the regioselective hydroxylation pathways. 5. Ortho- and meta halosubstituted biphenyls were less rapidly metabolised when compared with para substituted isomers.