Metabolism of benzo(a)pyrene by a dioxygenase enzyme system of the freshwater green alga Selenastrum capricornutum.

The green alga Selenastrum capricornutum was incubated with benzo(a)-pyrene under an atmosphere of 20% (18)O2: 80% N2. The cis-11,12-dihydro-11,12-dihydroxybenzo(a)pyrene, cis-7,8-dihydro-7,8-dihydroxybenzo(a)pyrene and cis-4,5-dihydro-4,5-dihydroxybenzo(a)pyrene, were isolated by HPLC and analyzed by mass spectrometry. The metabolites produced molecular ions at m/z 290 and 286. Elemental analysis of the ion at m/z 290 gave an elemental composition of C20H14(18)O2 with 13% (18)O2 incorporation. The results indicate that S. capricornutum produces cis vicinal dihydrodiols from molecular oxygen via a dioxygenase enzyme pathway. The dioxygenase enzymes are characteristic of the bacterial metabolic pathway and unlike those of eukaryotic organisms which involve monooxygenase enzymes.