The Cytochrome P-450 Substrate Optical Difference Spectra of Pesticides with Mouse Hepatic Microsomes by R. B. MAILMAN and ERNEST HODGSON Department of Entomology North Carolina State University Raleigh, N.C. 27607 The interaction of pesticides with microsomal oxidative enzyme systems in both target (HODGSON and PLAPP 1970; HODGSON 1968) and non-target organisms (FOURNIER 1970) has been under increased scrutiny. An important consideration in the use of pesticides is the effect they may have on non-target species. In mammals, the hepatic microsomal enzymes have been shown to be of primary importance in the oxidation of xenobiotics as well as being induced by many of these same compounds (HODGSON 1968; GILLETTE et al. 1969; CONNEY 1967). The oxidation of foreign compounds by microsomal enzymes is now thought to involve an initial binding to the oxidized form of cytochrome P-450 (REMMER et al. 1966). This complex can then accept electrons from NADPH by way of NADPH-cytochrome P-450 reductase and react with molecular oxygen to give a variety of oxidized products (HODGSON 1968). It has also been shown that compounds of a variety of different chemical structures bind to the oxidized cytochrome causing perturbations of the Soret optical spectrum of several characteristic types. The first type, known as type I, results in an optical difference spectrum with an absorption minimum at 416-420 nm and an absorption maximum at 385-390 nm (COOPER et al. 1965). The second type of perturbation, type II, has a maximum in the range of 424-435 nm and a minimum at 390-410 nm (SCHENKM Net al. 1967; IMAI and SATO 1966). A "type III" spectrum featuring double Soret peaks between 420 and 460 nm has been reported (PHILPOT and HODGSON 1971; OMURA and SATO 1964) but since it involves binding to the reduced form of cytochrome P-450, it will not be considered here. Compounds of a variety of structures have been shown to give type I binding (SASAME and GILLETTE 1969; IMAI and SATO 1969), whereas it has been stated that aromatic nitrogen compounds (T'SAI et al. 1970) aliphatic amines (JEFCOATE et al. 1969), alcohols (DIEHL et al. 1970) and certain other compounds (SATO et al. 1970; HEWICK and FOUTS 1970; ICHIKAWA and YAMAMO 1965) produce type II binding. METHODS AND MATERIALS Microsomes were prepared from the livers of six week old male mice from the North Carolina Department of Health strain, inbred since 1910. The mice were fed Purina Lab Chow and water ad libitum. The mice were killed by decapitation, the livers Bulletin of Environmental Contamination & Toxicology, Voh 8, No. 3, 1972, 9 by Springer-Verlag New York Inc. 186