2,3,7,8-Tetrachlorodibenzo-p-Dioxin: Covalent Binding of Reactive Metabolic Intermediates Principally to Protein in vitro

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is metabolized by the mouse liver cytochrome P-450-mediated monooxygenase system to reactive intermediates which bind covalently to cellular macromolecules. Although very difficult to quantitate, the presumably covalent binding to microsomal protein occurs between 120-2640 .times. more readily than binding to deproteinized DNA in the in vitro reaction. Because of the extremely high rate of binding to protein rather than to DNA, it is visualized that TCDD metabolites may be so reactive that they bind in or near the P-450-active site where the TCDD is monooxygenated. This extreme reactivity may preclude the formation of detectable quantities of phenols, dihydrodiols, or conjugated products. The rate of TCDD metabolism is estimated to be 9000-36000 .times. lower than the rate of P-450-mediated benzo[a]pyrene metabolism. This is the first demonstration that TCDD is metabolized in any organism. There remains the possibility that this covalently-bound radioactivity represents metabolites of contaminants, present in the radiolabeled TCDD sample in very minute amounts, rather than metabolites of tritiated TCDD itself. The possible relationship between P-450-mediated metabolism of this environmental contaminant and its extreme toxicity or teratogenicity is discussed.