Characterization of enzymes involved in biotransformation of polycyclic aromatic hydrocarbons in terrestrial isopods

Little is known about the capacity of terrestrial invertebrates to transfor m organic soil pollutants such as polycyclic aromatic hydrocarbons (PAHs). Studies were designed to characterize microsomal mixed function oxygenase a nd accompanying conjugation enzymes from the hepatopancreas of the terrestr ial isopods Porcellio scaber and Oniscus asellus using pyrene and hydroxypy rene as model substrates. The hydroxylation of pyrene and the formation of pyreneglucoside and pyrenesulfate appeared to be sensitive measures for the activity of cytochrome P450 aryl hydrocarbon hydroxylase (AHH), uridinedip hosphateglucosyl-transferase (UDPGT). and aryl sulfotransferase (ST), respe ctively. Treatment with the antibiotic riphampicine demonstrated that the e nzyme activities originate from the animals themselves and not from symbiot ic microflora present in the hepatopancreas and the gut. In both species. S T has a very high affinity for 1-hydroxypyrene with K-m values two orders o f magnitude lower than that of UDPGT. The V-max values of UDPGT, however, a re 10- to 20-fold higher than that of ST. Taking the P450 activities into c onsideration, both species are expected to transform PAHs in an equally eff ective way. When the isopods were fed with food containing benzo[a]pyrene a nd 3-methyl-cholanthrene, none of the enzyme activities appeared to be indu cible except for a small enhancement of UDPGT in O. asellus. Our findings i ndicate that terrestrial isopods have a high, noninducible capacity for bio transformation of PAHs and that the sulfate conjugation pathway is as impor tant as the carbohydrate conjugation pathway. This conclusion is consistent with the low body residues of parent PAHs found in the field.