Mutagen activation of 1,2‐dibromo‐3‐chloropropane by cytosolic glutathione s‐transferases and microsomal enzymes

Abstract

It is not clear whether glutathione (CSH) conjugation to 1,2‐dibromo‐3‐chloropro‐pane (DBCP) results in genotoxic activation. Therefore S9, cytosolic, and microsomal fractions from uninduced rat liver were evaluated for their relative ability to activate DBCP in a modified Ames system. The S9 enzymes, either alone or in combination with exogenous CSH, did not enhance the mutagenicity of DBCP; identical results were obtained with cytosolic enzymes. Significant mutagenic activation of DBCP was produced by either S9 or microsomal fractions in the presence of NADPH. Activation was proportional to cytochrome P‐450 concentrations, and was diminished by exogenous GSH. The protection against genotoxicity exerted by CSH did not require cytosolic glutathione S‐transferases (GST). Thus, mutagenic activation of DBCP as obtained with S9 fractions is primarily due to biotransformation by microsomal rather than by cytosolic enzymes. Kinetic studies of cytosol‐catalyzed conjugation of CSH to DBCP revealed tissue‐specific differences in apparent K_m and V_max. Renal and tes‐ticular CSTs were associated with 28–46% smaller V_max values when compared to hepatic CSTs (31.2 ± 7.9 nmol/min • mg protein). However, renal and testicular GSTs had relatively higher affinities for DBCP. Thus, extrahepatic tissues possess significant capacity to conjugate CSH to DBCP. DBCP‐CSH conjugates may undergo enzymatic modification by extrahepatic peptidases and β‐lyase to yield other sulfur‐containing moieties that perhaps mediate DBCP's extrahepatic toxicity.