Some carboxylic esterases of the pea (Pisum sativum L.)

Graduation date: 1968Although esterases have been reported in peas, their properties\ud have not been studied. The purpose of this investigation was to\ud determine the substrate and inhibitor specificity of the esterases\ud present in a water extract of lyophilized peas, and to determine if\ud pea esterases could be classified according to the criteria established\ud for animal esterases.\ud Esterase activity was determined manometrically using the\ud Gilson differential respirometer. The effect of pH on esterase\ud activity was determined using phenyl acetate, phenyl propionate,\ud tripropionin, and tributyrin as substrates. The pH optima appeared\ud to lie between 6.9 and 7.2, and pH 7 was selected for use in this\ud study. The acetyl, propionyl, and n-butyryl esters of phenol,\ud 2-naphthol-6-SO₃ Na and glycerol were hydrolyzed by the enzyme\ud extract. Long chain esters of 2-naphthol-6-SO₃Na, however, were not hydrolyzed. Cholinesterases and lipases did not appear to be\ud present in the extract since only a very small amount of activity\ud was observed when the choline esters and triolein were used as\ud substrates.\ud Using phenyl propionate and phenyl butyrate as substrates,\ud esterase activity, based on the original extract, decreased with\ud dilution. Later work revealed that the esterase(s) which hydrolyzed\ud phenyl propionate were inhibited by heavy metal ions and activated\ud by metal complexing agents. Hence, a possible explanation for the\ud decrease in activity was inhibition by metal ions in the distilled\ud water.\ud The effects of the inhibitors parathion, tetraethyl pyrophosphate,\ud and diisopropyl phosphorofluoridate at concentrations ranging from 10⁻¹ M to 10⁻¹⁰ M on esterase activity were determined.\ud The data suggested the presence of as many as six esterases in the\ud aqueous extract of peas, three for which the evidence was quite conclusive.\ud On the basis of their inhibition by organophosphorus compounds,\ud all but one of the esterases appeared to be of the B type.\ud Physostigmine sulfate (10⁻⁵ M) had no effect on esterase\ud activity with the nine substrates used indicating that the activity was\ud not due to cholinesterases.\ud At least one of the esterases which hydrolyzed each of the\ud substrates was sensitive to 10⁻³ M p-chloromercuribenzoic acid sodium salt suggesting the importance of sulfhydryl groups for\ud enzyme activity