Abstract
Carcinogens are metabolised by the mixed-function oxidase system of the endoplasmic reticulum, which results in the formation of reactive metabolites. These products exert a toxic effect upon the endoplasmic reticulum, causing changes in the structure and function of the membranes. The degranulation 'in vitro' of the membranes by carcinogens has been studied. Two methods of assay were compared, and one used for further studies. The extent of degranulation varied between different preparations of membranes, and it was not found possible to correlate changes in drug-metabolising enzyme activities with degranulation in vitro. It was shown that the density (and degree of granulation) of membranes was variable, and that the density profiles were little affected by inducing agents. Differences between different membrane preparations can be due to slight differences in the preparative media. The distribution of cytochrome P-450 species within the endoplasmic reticulum was studied, and it was shown that cytochrome P-450 is found predominantly in 'rough' membranes, whilst cytochrome P-448 is a 'juvenile' form found in neonates and also 'smooth' membranes. The effect of inducing agents, and carcinogens on the cytochromes of 'rough' and 'smooth' membranes was also studied. Sex differences in drug metabolising enzymes indicate that cytochrome P-448 is also found in female rats, although the activities are low. Cytochrome P-450 develops from cytochrome P-448 in males during puberty. The induction of cytochromes P-450 and P-448 in male rats was studied, and it was shown that whereas the increase in enzyme activities paralleled the increase in cytochrome P-450, increased enzyme activities were observed before detectable increases in cytochrome P-448. The appearance of cytochrome P-448 brought about by treatment with carcinogens was studied during inhibition of protein synthesis. It was shown that 'de novo' protein synthesis was not necessary for the appearance of the altered enzyme activities associated with cytochrome P-448. Inhibitors of cytochrome P-450 were used 'in vitro', and the effects upon enzyme activities in induced membranes compared. Further kinetic studies of ethoxycoumarin O-deethylase were performed after noting a stimulation of. activity. The results suggest that the activity is not solely mediated by the cytochrome as a terminal oxygenase, and that an alternative mechanism of oxygenation may be present. The results are discussed in relation to the overall mechanism of carcinogenicity, involving changes in the membrane structure.