Conjugation of benzo[ a]pyrene metabolites by freshwater green alga Selenastrum capricornutum

Abstract

Benzo[ a]pyrene (BaP) undergoes metabolic transformation in mammals via oxidative, hydrolytic, and conjugative processes; however, little is known concerning BaP conjugation in freshwater algae. It has been shown in this laboratory that BaP is metabolized by Selenastrum capricornutum via a dioxygenase pathway. This study describes the conjugation of BaP metabolites by a green alga, Selenastrum capricornutum. Cultures were exposed to 1160 μg/l [ 14C]BaP for 4 days at 23°C under gold fluorescent lights on a diurnal cycle of 16 h light, 8 h dark. Of the total metabolites in the algal culture, 89% were present in media. BaP and non-conjugated metabolites were separated from conjugated metabolites by chromatography on neutral alumina columns using solvents of increasing polarity. Seventy-one percent of the BaP metabolites were conjugates of which 12.2%, 12.0% and 12.4% were sulfate ester and α- and β-glucose conjugates, respectively. Conjugates that coeluted with sulfate esters were hydrolyzed with arylsulfatase, α- or β-glucosidase; high performance liquid chromatography (HPLC) analysis indicated that the major product of each enzymatic hydrolysis was the 4,5-dihydrodiol (87.2, 69 and 53%, respectively). Eighty-six percent of the conjugates were acid labile following incubation for 2 h in 4 N HCl at 37°C. To our knowledge this is the first demonstration of the metabolism of a polynuclear aromatic hydrocarbon by a freshwater green alga through a dioxygenase pathway and subsequent conjugation and excretion.