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Treatment of isolated rat liver mitochondria with certain tetrachlorobiphenyls (TCBs) resulted in uncoupling of oxidative phosphorylation. In the present study, we examined the effects of chloro-substituent sites in TCB on the uncoupling action. Furthermore, the mechanism by which effective TCBs induce uncoupling action was explored. 2, 3, 2, 3'- (23-), 2, 4, 2, 4'- (24-), 2, 5, 2, 5'- (25-), and 2, 6, 2, 6'- (26-) TCBs caused uncoupling of oxidative phosphorylation, whereas 3, 4, 3, 4'- (34-) TCB did not. TCBs which were effective in causing uncoupling action contained chlorine atoms at the 2, 2'-positions in the biphenyl ring. 2, 2'-Chloro substitutions give the molecule a highly angular (non-planar) conformation. On the other hand, 34-TCB, which was ineffective, possesses chlorine atoms at lateral adjacent positions in the biphenyl ring. This substitution pattern allows a coplanar conformation of the molecule. Thus, the conformation of effective TCBs was non-planar, whereas that of ineffective isomers was coplanar. The permeability of mitochondrial membranes to ions was increased by the effective TCBs (23-, 24-, 25-, and 26-TCBs) as evidenced by K+-and Ca2+-release from mitochondria, which was followed by the dissipation of membrane potential. However, the ineffective TCB (34-TCB) neither increased the ion-permeability nor dissipated the membrane potential. These facts led us to propose to the following uncoupling mechanism; the effective TCBs, when intercalated into the mitochondrial membranes, produce non-specific increases in membrane permeability to ions, which leads to the dissipation of membrane potential. These effects of non-planar TCBs appear to be different from the mechanism of protonophoric uncouplers in which the dissipation of membrane potential is performed by only H+ transfer across the mitochondrial membranes.

chemical and pharmaceutical bulletin/chemical and pharmaceutical bulletin

Studies of the influence of chloro-substituent sites in tetrachlorobiphenyls on the uncoupling of oxidative phosphorylation in isolated rat liver mitochondria.