ARYL-HYDROCARBON HYDROXYLASE-ACTIVITY IN SPONTANEOUS MAMMARY-TUMORS IN RAT

The development of spontaneous mammary tumors was observed for about 2 years in a group of 25 female Sprague-Dawley rats aged over 1 year at the beginning of the study. All younger females in our animal facilit y were similarly monitored. In old females, the incidence of spontaneo us mammary tumors was 64%. The parity of rats did not protect them fro m tumorigenesis, but the proportion of malignant tumors was higher in virgin (57%) than in parous (13%) rats. Activities of cytochrome P(450 )IA(1)-dependent enzyme (aryl hydrocarbon hydroxylase, AHH) and NADPH- cytochrome P-450 reductase (NCR) were determined in microsome fraction s isolated from livers, lungs, uteri and tumors of rats. AHH and NCR a ctivities in tumors and uteri were low compared to those in livers or lungs. In tumors, the activity distributions were wide, even in differ ent tumors of the same animal the AHH activities varied as widely as b etween different animals. The activities in benign and malignant tumor s were not statistically significantly different. No correlation with liver, lung or uterine activities was found either. With ageing of the rat, the AHH activities in tumors, liver and lungs decreased. The beh avior of AHH in spontaneous mammary tumors in rats seems to be similar to that found in chemically induced tumors and seems to show individu al regulation, possibly altered by tumorigenesis in each individual tu mor.