Relative biochemical reactivity of three hexachlorocyclohexane isomers

The rate and mechanisms of degradation of three hexachlorocyclohexane (HCH) isomers were studied with rat liver cytochrome P450, which is known to react with chlorinated compounds in the presence and absence of molecular oxygen. In addition, P450 is produced from HCH metabolites which are found aerobically and anaerobically during microbial biodegradation of this compound in a complex soil system. Conversion assays with P450 were carried out with alpha-, beta-, and gamma-HCH. Hexachloroethane served as a reference compound for bioconversion kinetics. Under anaerobic conditions, gamma-HCH was readily dechlorinated (r0 = 0.31 nmol/min.nmol cyt.P450) to tetrachlorocyclohexene and monochlorobenzene. gamma-HCH was also reductively dechlorinated, but at a much slower rate (r0 = 0.03 nmol/min.nmol cyt.P450). Both substrates followed Michaelis-Menten kinetics (gamma-HCH: Vmax = 1.87 nmol/min.mg and Km = 47 mumol/liter; gamma-HCH: Vmax = 0.24 nmol/min.mg and Km = 201 mumol/liter). Under aerobic conditions, bioconversions of gamma-HCH (r0 = 0.05 nmol/min.nmol cyt. P450) and alpha-HCH (r0 smaller than detection limit) were slower. beta-HCH proved to be recalcitrant under both aerobic and anaerobic conditions. The differences in bioconversion rates between the three HCH isomers could be related to the differences in spatial chlorine configuration of the three isomers. beta-HCH recalcitrance was explained by the absence of axial-orientated chlorines.