HYDROLYSIS OF CARBONATES, THIOCARBONATES, CARBAMATES, AND CARBOXYLIC ESTERS OF ALPHA-NAPHTHOL, BETA-NAPHTHOL, AND P-NITROPHENOL BY HUMAN, RAT, AND MOUSE-LIVER CARBOXYLESTERASES

Thirty carbonates, thiocarbonates, carbamates, and carboxylic esters o f alpha-naphthol, beta-naphthol, and p-nitrophenol were synthesized an d tested as substrates for liver carboxylesterases from the crude micr osomal fractions of human and mouse, and purified isozymes, hydrolases A and B, from rat liver microsomes. The carbonates, thiocarbonates, a nd carboxylic esters of alpha-naphthol were cleaved more rapidly than the corresponding beta-naphthol isomers by the mammalian liver esteras es. Alpha-Naphthyl esters of acetic, propionic, and butyric acids were among the best substrates tested for these enzymes. The majority of t he substrates was consistently hydrolyzed at higher rates by hydrolase B compared with hydrolase A, although the Michaelis-Menten constant ( K(m)) values of selected substrates differed widely with these two iso zymes. Malathion was a 15-fold better substrate for hydrolase B than f or hydrolase A. Compared with the corresponding carboxylates, the carb onate moiety of alpha- and beta-naphthol and p-nitrophenol lowered the specific activities of the enzymes by about fivefold but improved sta bility under basic conditions. The optimum pH of mouse liver esterase with the acetate, methylcarbonate, and ethylthiocarbonate of alpha-nap hthol was between pH 7.0 and pH 7.6. Human and mouse liver microsomal esterase activities were about five orders of magnitude lower than the esterase activities of purified rat liver hydrolase B. A relationship between the catalytic activity of the enzymes and the lipophilicity o f the naphthyl substrates indicated that (i) in the alpha- and beta-na phthyl carbonate series, an inverse relationship between enzyme activi ty and lipophilicity of the substrates was observed, whereas (ii) in t he alpha-naphthyl carboxylate series, an increase in enzyme activity w ith increasing lipophilicity of the substrates up to a logP value of a bout 4.0 was observed, after which the enzyme activity decreased.