Arachidonic acid-dependent activation of benzo[a]pyrene to bind to proteins with cytosolic and microsomal fractions from rat liver and lung.

Activation of benzo[a]pyrene to bind to proteins by co-oxidation with prostaglandin synthesis was studied in rat liver and lung microsomes and cytosols. The kinetics of this activation showed a two-phase reaction; a rapid initial reaction for 2-4 min after addition of arachidonic acid and then a slow reaction or a plateau state. The reaction was linear only with low contents of the enzyme proteins, and was slower with more than 1 mg of enzyme per ml of incubation mixture. Other unsaturated fatty acids were also effective in activating benzo[a]pyrene with both microsomes and cytosols. With microsomal proteins linoleic acid was more effective than arachidonic acid, whereas with cytosolic proteins arachidonic acid was the best cofactor. Linolenic acid could also activate benzo[a]pyrene, though less efficiently, but oleic acid had no influence on the binding. Indomethacin did not inhibit the activation, but nordihydroguaiaretic acid and quercetin significantly reduced the binding. Addition of hematin significantly increased the binding. The NADPH-dependent bindings of benzo[a]pyrene to proteins with liver and lung microsomes were one-third and one-twelfth the values after incubation with arachidonic acid. Addition of glutathione or Ca2+ ion reduced the binding significantly. The present results suggest the importance of co-oxidation with lipoxygenase for activation of benzo[a]pyrene and the possible role of both the arachidonic acid cascade system and the NADPH-dependent cytochrome P-450 system in metabolic activation of chemical carcinogens.