IN-VITRO METABOLISM OF PYRIPROXYFEN BY MICROSOMES FROM SUSCEPTIBLE AND RESISTANT HOUSEFLY LARVAE

Levels of cytochrome P450 and be were investigated in microsomal enzym es of houseflies from the gut and fat body of the third instar larvae of a pyriproxyfen-resistant strain (YPPF) and two pyriproxyfen-suscept ible strains (YS and SRS). In comparison to the YS and SRS strains, YP PF microsomes had higher levels of total cytochrome P450s in both the gut and fat body. Furthermore, microsomes from the gut and fat body of YPPF larvae were found to have a much greater ability to hydroxylate aniline than YS larvae. In vitro metabolism studies of pyriproxyfen in dicated that the metabolic rates were much higher in both the gut and fat body of YPPF larvae than of YS and SRS larvae. The major metabolit es of pyriproxyfen in houseflies were identified to be 4'-OH-pyriproxy fen and 5''-OH-pyriproxyfen. Cytochrome P450 inhibitors, piperonyl but oxide (PB) and 2-propynyl 2,3,6-trichlorophenyl ether (PTPE), decrease d the metabolic rates significantly in all three strains. This study c onfirmed that microsomal cytochrome P450 monooxygenases play an import ant role in the pyriproxyfen resistance of the housefly. Furthermore, it suggests that the fat body must be as important as the gut for the metabolism of pyriproxyfen in resistant housefly larvae. (C) 1998 Wile y-Liss, Inc.