Ali B, Kaur S, James E C, Parmar S S
Biochem Pharmacol. 1985 Jun 1;34(11):1881-6. doi: 10.1016/0006-2952(85)90302-8.
The present study has provided evidence for the existence of three distinct carboxylesterases involved in the hydrolysis of steroid esters, where two enzymes are possibly responsible for the metabolism of hydrocortisone hemisuccinate (HCHS) at pH 5.5 and 8.0, and a third enzyme for the metabolism of hydrocortisone acetate (HCAC) at pH 8.0, in isolated rat liver microsomes. The activity of all three enzymes in rat liver was induced significantly by the administration of phenobarbital while no such function in enzyme activity was observed in animals receiving 3-methylcholanthrene or benzo[a] pyrene under similar experimental conditions. The increase in the activity of HCHS esterase I (HCHS-E1) active at pH 5.5, HCHS esterase II (HCHS-E2) active at pH 8.0, and HCAC esterase (HCAC-E) was approximately 7 to 8, 3- and 3-fold respectively. On the other hand, the degree of induction of nonspecific microsomal carboxylesterase acting on p-nitrophenylacetate (PNPA) was significantly less. The Km values for the hydrolysis of HCHS at pH 5.5 and 8.0 and HCAC by rat liver microsomes obtained from control rats were 2.45, 2.02 and 1.6 mM, respectively, and these Km values were not changed significantly in preparations obtained from rats treated with phenobarbital. The distinct in vitro responses displayed by hepatic microsomal steroid esterases to various inhibitors were able to distinguish three different enzymes which also differed from nonspecific carboxylesterases. The activity of HCAC-E was inhibited by NaAsO2 and AgNO3 while that of HCHS-E1 and HCHS-E2 remained unaffected. Selective inhibition of HCHS-E1 by NaF, HgCl2 and p-chloromercuribenzoate and that of HCHS-E2 by NiSO4 indicated the possible existence of different enzymes or isozymes of a carboxylesterase catalyzing HCHS hydrolysis. The effects elicited by the inhibitors on the activity of PNPA esterase were different from those observed with steroid esterases. Furthermore, the present study has also indicated species variations in the distribution of steroid esterases in the livers of rat, mouse, dog and cat.
本研究为参与甾体酯水解的三种不同羧酸酯酶的存在提供了证据。在分离的大鼠肝微粒体中，两种酶可能负责在pH 5.5和8.0条件下氢化可的松半琥珀酸酯（HCHS）的代谢，第三种酶负责在pH 8.0条件下醋酸氢化可的松（HCAC）的代谢。给予苯巴比妥可显著诱导大鼠肝脏中所有这三种酶的活性，而在类似实验条件下，接受3-甲基胆蒽或苯并[a]芘的动物未观察到酶活性有此类变化。在pH 5.5有活性的HCHS酯酶I（HCHS-E1）、在pH 8.0有活性的HCHS酯酶II（HCHS-E2）以及HCAC酯酶（HCAC-E）的活性增加分别约为7至8倍、3倍和3倍。另一方面，作用于对硝基苯乙酸（PNPA）的非特异性微粒体羧酸酯酶的诱导程度明显较低。从对照大鼠获得的大鼠肝微粒体在pH 5.5和8.0条件下对HCHS以及对HCAC水解的Km值分别为2.45、2.02和1.6 mM，在用苯巴比妥处理的大鼠制备物中这些Km值没有显著变化。肝微粒体甾体酯酶对各种抑制剂表现出的不同体外反应能够区分三种不同的酶，它们也不同于非特异性羧酸酯酶。HCAC-E的活性受到亚砷酸钠和硝酸银的抑制，而HCHS-E1和HCHS-E2的活性不受影响。氟化钠、氯化汞和对氯汞苯甲酸对HCHS-E1的选择性抑制以及硫酸镍对HCHS-E2的选择性抑制表明可能存在催化HCHS水解的不同酶或羧酸酯酶同工酶。抑制剂对PNPA酯酶活性产生的影响与甾体酯酶所观察到的不同。此外，本研究还表明大鼠、小鼠、狗和猫肝脏中甾体酯酶的分布存在种属差异。
