Debenzoylating and deacetylating activities of rat liver and mammary gland microsomes. Effect of ovariectomy.

This study compared the rates of N-deacylations of N-hydroxy-N-2-fluorenylbenzamide (N-OH-2-FBA) with those of its analogue, N-hydroxy-N-2-fluorenylacetamide (N-OH-2-FAA), by the mammary gland (tumor target for both compounds) and the liver of female Sprague-Dawley rats and examined the effect of ovariectomy on these activities. N-Debenzoylation of N-OH-2-FBA was catalyzed by the mammary and liver microsomes of 50-day-old female rats at similar rates (approximately 24 nmol/min/mg). The activity of both tissues increased (up to 1.8 times) after ovariectomy at 42, 32 and, especially, 22 days of age. The rapid hydrolysis appeared to be unique for the benzoyl group since N-OH-2-FAA was deacylated only approximately 0.05 and 0.004 times as fast by the liver and mammary microsomes, respectively, and these low rates were unaffected by ovariectomy. Since such substrate specificity would be of significance in the metabolism of xenobiotics and drug design, esterase activity and its sensitivity to ovariectomy at 22 days of age were examined with several acetylated and benzoylated substrates in the liver and mammary microsomes and compared with those of male liver. Tissues of rats of both sexes had a greater capacity to hydrolyze carboxyl esters than amides. Expect for N-2-fluorenylacetamide (2-FAA) and o-nitrophenylacetate (o-NPA), all substrates were hydrolyzed by liver microsomes of the male up to 3.9 times faster than by those of the female. Microsomes of female liver hydrolyzed acetylated substrates 1.2 to 25 times faster than benzoylated analogues except for N-OH-2-FBA and benzamide. By contrast, mammary gland microsomes hydrolyzed benzoylated compounds 1.4 to 333 times faster except for 2-naphthyl benzoate. Respective rates of hydrolysis of o-NPA by microsomes of liver and mammary gland were 1.7 and 0.6 times those of p-NPA. After ovariectomy, deacylating activities increased (up to 1.6 times) except for those of 2-FAA and acetanilide. All deacylations were > 98% inhibited by 0.1 mM paraoxon, indicating catalysis by serine hydrolases. The results suggest involvement of multiple carboxylesterases and indicate that certain benzoylated xenobiotics may have a greater effect on the mammary gland than acetylated xenobiotics because of their greater vulnerability to hydrolysis by esterases of mammary gland.