Deacetylation of cinobufagin by rat liver
Zhang, L.; Aoki, K.; Yoshida, T.; Kuroiwa, Y.
Drug Metabolism and Disposition the Biological Fate of Chemicals 20(1): 52-55
1992
ISSN/ISBN: 0090-9556 PMID: 1346996 Document Number: 396058
The enzyme system mediating the deacetylation of cinobufagin (CB) at the 16-position to give deacetylCB was characterized in the rat. Tissue distribution studies showed that the highest activity of CB deacetylation was mainly localized in the liver microsomal fraction. Some activity was also detected in the serum and intestine. Kinetic studies of the enzymatic reaction carried out by microsomes demonstrated that the formation of deacetyl-CB increased linearly with time up to 60 min and with protein content up to 10 mg. Apparent Km and Vmax values calculated from Lineweaver-Burk plots were 2.7 .times. 10-4 M and 4.17 nmol/mg of protein/min, respectively. Low concentration of several metal salts (AgNO3, MgCl2, CoSO4, and CuCl2) did not affect CB deacetylase activity. Microsomal CB deacetylation was inhibited by the organophosphorus compounds cyanox and fenitrothion at 1.0 .times. 10-6 M. Eserine sulfate, disulfiram, rifampicin, and phenacetin at 1.0 .times. 10-4 M also decreased CB deacetylase activity. Aspirin, sodium fluoride, and EDTA at 1.0 .times. 10-3 M did not inhibit the deacetylation. In vivo treatment of rats with phenobarbital resulted in a 2-fold increases in microsomal CB deacetylase activity. All of these results suggest that the enzyme responsible for CB deacetylation is somewhat different from the other characterized esterases.