STUDIES ON THE CLASSIFICATION OF THE ENZYMES HYDRO-LYZING ESTER-FORM DRUGS IN LIVER MICROSOMES

Abstract

The drug-metabolizing enzymes catalyzing such reactions as oxidation, reduction, conjugation and hydrolysis have been reported to be localized mainly in liver microsomes (1, 2). Microsomes may also be considered as hydrolytic particles (3). The enzymes hydrolyzing ester-form drugs, e.g. aspirin (4-6), procaine (7), cocaine (8-11), atropine (8, 12, 13) and choline-esters (14, 15) have also been shown to be in the liver and some other tissues, but little is known 'about the properties and intracellular localization of these esterases as compared with cholinesterase which is closely related to synaptic transmission. It appears of interest to investigate drug-metabolizing enzymes in liver microsomes further to elucidate their pharmacological actions and mechanisms of detoxication. As a fundamental classification of esterases, Aldridge (16) showed that 10-5 M organophosphorous compounds inhibit many enzymes possessing carboxylic esterase activity while other esterases are unaffected. He has named the latter A-type and the former B-type esterase. Neither A nor B-type are sensitive to 10-5 M eserine. Cholinesterase is inhibited completely both by 10-6 M organophosphates and by 10-5 M eserine. Subsequently A and B-types and cholinesterase in many vertebrate plasmata were separated electrophoretically by Goutier (17) and Augustinsson (18). Microsomal esterases have not been examined with this technique. The present paper describes experiments on the classification of esterases, especially drug-hydrolyzing enzymes, in liver microsomes of some rodents. The microsomes “solubilized” with sodium deoxycholate were fractionated chromatographically or electrophoretically, and the sensitivities of the esterases in each fraction to inhibitors and their kinetics were investigated.