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 facility were similarly monitored. In old females, the incidence of spontaneous mammary tumors was 64%. The parity of rats did not protect them from tumorigenesis, but the proportion of malignant tumors was higher in virgin (57%) than in parous (13%) rats. Activities of cytochrome P450IA1-dependent enzyme (aryl hydrocarbon hydroxylase, AHH) and NADPH-cytochrome P450 reductase (NCR) were determined in microsome fractions isolated from livers, lungs, uteri and tumors of rats. AHH and NCR activities in tumors and uteri were low compared to those in livers or lungs. In tumors, the activity distributions were wide, even in different tumors of the same animal the AHH activities varied as widely as between different animals. The activities in benign and malignant tumors 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 behavior of AHH in spontaneous mammary tumors in rats seems to be similar to that found in chemically induced tumors and seems to show individual regulation, possibly altered by tumorigenesis in each individual tumor.