The differentiation of the A-type esterases in sheep serum

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Abstract
The substrate-specificity pattern of paraoxonase preparations purified approximately 300 times has been compared with that of sheep serum. Paraoxonase did not hydrolyze phenyl acetate, whereas sheep serum hydrolyzed the compound at a high rate. The high phenyl acetate-hydrolyzing activity of sheep serum and the results of heat-inactivation experiments . suggested the presence of an A-type arylesterase in sheep serum distinct from paraoxonase. The ratio of p-nitrophenyl acetate hydrolysis to paraoxon hydrolysis decreased from 67 [plus or minus] 7 in serum to 9 [plus or minus] 4 in purified preparations. Heat inactivated the p-nitrophenyl acetate- and p-nitrophenyl butyrate-hydrolyzing activity of sheep serum at a significantly slower rate than it did the paraoxon-hydrolyzing activity. From this and supporting evidence involving the Km for hydrolysis activity of sheep, rabbit and hog sera and of purified paraoxonase towards p-nitrophenyl acetate, it was concluded that a new esterase hydrolyzing p-nitrophenyl acetate, but not paraoxon, existed in sheep and probably in hog serum. The esterase was called D-esterase. Evidence was presented which suggested that D-esterase was not identical with the phenyl acetate-hydrolyzing arylesterase or with the C-esterase of hog kidney. The effect of manganese and the decrease in the diisopropyl phosphorofluoridate-hydrolyzing to paraoxon-hydrolyzing rate from 1.8 in serum to 0.4 in purified paraoxonase preparations suggested the presence in sheep serum of a diisopropyl phosphoro-fluoridatase similar to that of hog kidney, in addition to paraoxonase. The paraoxonase in sheep serum was responsible for most of the tabun hydrolysis. Sheep-serum and purified paraoxonase preparations hydrolyzed m-nitrophenyl acetate more rapidly than they hydrolyzed p-nitrophenyl acetate, and o-nitrophenyl acetate was hydrolyzed least rapidly. Purified paraoxonase did not hydrolyze tetraethyl pyrophos-phate.