Aroclor 1254 and
fireMaster BP-6, two commercial
polychlorinated biphenyl (PCB) and polybrominated
biphenyl (PBB) preparations, exhibit comparable
biologic and toxic effects. In the present study the commercial PBB was more active than
Aroclor 1254 in causing thymic
atrophy in male Wistar rats. However, a direct comparison of the relative effects of
bromine vs
chlorine substituents is not possible with the commercial PBB and PCB mixtures due to their complex congeneric composition. This study reports the synthesis and
biologic and toxic effects of a series of laterally substituted 3,3',4,4'-tetrahalobiphenyls which contain the following variable molecular Cl/Br ratios; Br4, Br3Cl, Br2Cl2 (two isomers), BrCl3, and Cl4.
3,3',4,4'-Tetrabromobiphenyl and 3,4,4'-tribromo-3'-chlorobiphenyl (150 mumol/kg)-pretreated animals significantly inhibited the growth rate of and caused thymic
atrophy in immature male Wistar rats whereas those isostereomers with reduced Br (and increased Cl) content were either less active or inactive. Pretreatment of male Wistar rats with 10 mumol/kg of the 3,3',4,4'-tetrahalobiphenyls and determination of their effects as inducers of the hepatic microsomal
drug-metabolizing
enzymes also illustrated the effects of the relative Cl/Br ratios on induction potencies. Both 3,3',4,4'-tetrabromo- and 3,4,4'-tribromo-3'-chlorobiphenyl maximally induced the
cytochrome P-448-dependent
monooxygenases,
benzo[a]pyrene and
4-chlorobiphenyl hydroxylase; the order of potency of the other isostereomers was 4,4'-dibromo-3,3'-dichloro- congruent to 3,4-dibromo-3',4'-dichlorobiphenyl greater than 4-bromo-3,3',4'-trichloro- greater than
3,3',4,4'-tetrachlorobiphenyl. With few exceptions this order of potency was observed for the induction of
benzo[a]pyrene hydroxylase and
ethoxyresorufin O-deethylase in rat
hepatoma cells in culture and for their relative binding affinities to the rat cytosolic receptor
protein. The data clearly demonstrate that the
biologic activities of this group of isosteric halogenated biphenyls are enhanced with increasing
bromine substitution and also support the hypothesis that the activities of this class of chemicals are mediated through the receptor.