An Endogenous Inhibitor ofMicrosomalMixed-Function Oxidasesin Homogenates oftheSouthern Armyworm(Prodenia eridania)

Itisnowgenerally accepted thatinsects metabolize insecticides andother foreign compounds by meansofmicrosomal enzymes, whichappear tobe biochemically similar tothose foundinmammalian liver (Smith, 1967;Terriere, 1968). Ininvestigations ofthemicrosomal enzymesofinsect species, however, amajorproblem arises fromtherelatively smallsizeoftheorganism andtheconsequently small amountsoftissue available. Toobviate this problem ithasbecomecommonpractice toemploy homogenates orcentrifugal fractions obtained from wholeinsects. Enzymeactivity inthese'whole preparations' isusually lowandshort-lived compared withtheactivity insimilar fractions ofmammalian liver, anditishighly questionable whether suchpreparations provide a realistic measureof themetabolic activity oftheinsect underinvestigation. Themicrosomal enzymesresponsible fordrug andinsecticide metabolism- arenotequally distributed throughout theinsect, butarelocalized inthetissues ofspecific organs. Thusmaximal epoxidase activity inthelarvae ofthesouthern armyworm(Prodenia eridania) isfoundinthe tissues ofthemid-gut (Krieger WChakraborty, Sissons & Smith,1967).Suchnatural inhibitors havebeennotedby several workers studying themicrosomal enzymesofinsects and clearly haveconsiderable significance intheresults ofsuchinvestigations. Homogenatesofwhole houseflies arefoundtocontain heat-stable diffusible materials thatareinhibitory towards theactivity invitro ofmicrosomal preparations fromboth insects andmammals(Matthews &Hodgson, 1964).