Lake, BG; Collins, MA; Harris, RA; Stubberfield, CR; Caciola, P; Gangolli, SD
Many compounds have been found to stimulate hepatic xenobiotic-metabolising enzyme activities in experimental animals (Conney, 1967). Studies have demonstrated that the terminal oxygenase of microsomal xenobiotic metabolism, namely cytochrome P-450, consists of a number of distinct haemoproteins (Warner et al., 1978) and that these cytochromes respond differently to the administration of enzyme inducers (Haugen et al., 1976). However, some species differences in the response to these agents has been reported. For example, Thorgeirsson et al. (1979) have described differences in the response of the rat, two strains of mice, the rabbit and the guinea pig to induction by 20- methylcholanthrene. In the present communication we report the effect of three known inducers of rat mixed-function oxidase enzymes on hepatic xenobiotic metabolism in the guinea pig. Male Dunklin-Hartley guinea pigs (230-27Og) were used in the present studies. They were allowed free access to food and water. Aroclor 1254 (20mg/kg), hexachlorobenzene (30mg/kg) and 8-naphthoflavone (40mg/kg) were administered by daily intraperitoneal injections for 7 days. Control animals received corresponding quantities of the corn-oil vehicle (5 ml/kg). At 24 h after the last dose of the compounds the animals were killed by cervical dislocation and the livers immediately excised. Whole liver homogenates were prepared in 0.154~-KCI containing SOmM-Tris/HCI buffer, pH 7.4, and postmitochondrial supernatant and washed microsomal fractions were obtained by differential centrifugation. Postmitochondrial-supernatant fractions were assayed for activities of ethylmorphine N-demethylase (Holtzman et al.. 1968), 7-ethoxycoumarin 0-de-ethylase (Ullrich & Weber, 1972) and aniline 4-hydroxylase (Nakanish’i el al., 1971). Cytochrome P-450 (Omura & Sato, 1964) and protein (Lowry et al., 195 1) were determined in washed microsoma1 fractions. Of the three foreign compounds studied, only treatment with the polychlorinated biphenyl mixture Aroclor 1254 enhanced the relative liver weight of guinea pigs (Table 1). However, all three compounds induced the activities of the three mixed function oxidase enzymes studies and the microsomal content of cytochrome P-450. Aroclor 1254 and /3-naphthoflavone, but not hexachlorobenzene, also increased the liver microsomal protein content. The results of the present studies indicate a number of differences between the response of the guinea pig and the rat to the administration of known inducers of mixed-function oxidase enzymes. For example, only Aroclor 1254 treatment increased liver size in the guinea pig, whereas Aroclor 1254 (Stonard & Greig, 1976) and hexachlorobenzene (Stonard & Nenov, 1974) have been reported to increase the relative liver weight in the rat. In addition, /3-naphthoflavone is classified as a polycyclic-hydrocarbon-type inducer in the rat. (Boobis et al., 1977) and inducers of this class do not stimulate ethylmorphine N-demethylase activity in the rat (Alvares & Kappas, 1977; Sladek & Mannering, 1969). Another parameter used to assess the type of induction of hepatic microsomal xenobiotic metabolism is to determine the wavelength maximum of the dithionite-reduced carbon monoxide absorption complex with microsomal cytochrome P-450. Although Aroclor 1254 (Alvares & Kappas, 1977), hexachlorobenzene (Stonard & Greig, 1976) and 8-naphthoflavone (Boobis el al., 1977) all produce hypsochromic shifts in the wavelength maximum of cytochrome P-450 in the rat, we were unable to detect any marked spectral shifts in the guinea pig after treatment with any of these compounds. In conclusion, the results of the present studies indicate that the response of the guinea-pig hepatic microsomal mixed function oxidase complex to a number of enzyme inducers differs from that observed in the rat.