BIOTRANSFORMATION OF ORGANOPHOSPHORUS COMPOUNDS BY RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) LIVER IN RELATION TO BIOCONCENTRATION

Biotransformation of a series of organophosphorus compounds by the 9,0 00-g supernatant of rainbow trout (Oncorhynchus mykiss) liver was test ed in an in vitro system fortified either with NADPH-generating cofact ors or with reduced glutathione (GSH). Elimination rate constants for both systems were calculated from linear decay curves when substrate c oncentrations were used that were considerably lower than the K(m) val ues of the concerned enzymatic reactions. The results reveal a large v ariation in both the oxidative and the glutathione-mediated biotransfo rmation rate of the organophosphorus compounds. Half-lives ranged from 25 to 1,216 min in the NADPH system and from 18 to 381 min in the GSH system. Elimination rate constants in the GSH system were related to Hammett sigma constants or reactivity toward 4-nitrobenzylpyridine, wh ich substantiates the assumption that electrophilicity is the controll ing variable for the reaction with GSH within this particular class of compounds. A remarkable analogy was observed between compounds that w ere metabolized relatively quickly by glutathione S-transferases and c ompounds that showed a reduced bioconcentration factor in guppies. A s ignificant improvement of the relationship between the bioconcentratio n factor in guppies and the octanol/water partition coefficient was ob tained when the rate constant with GSH was introduced in this relation ship. Such an improvement was not obtained with the rate constants fro m the oxidative system. These observations are discussed in view of th e differences in the activities of the involved enzyme systems in the test species and in view of the possible relevance of the different bi otransformation pathways for the in vivo situation.