Abstract

Evidence is presented that human serum contains a single enzyme with both paraoxonase and arylesterase activities. Throughout the steps of purification and after obtaining over 600-fold purification of the enzyme, the arylesterase activity (measured with phenylacetate as the substrate) co-eluted and retained the same ratio of activity to paraoxonase activity as it had in the initial plasma sample. Paraoxon and DFP (diisopropylfluorophosphate) both complete with phenylacetate as substrates; the inhibition is of mixed type with paraoxon and competitive with DFP. Paraoxonase and arylesterase activities require calcium, and both are inhibited to the same degree by EDTA. Purified arylesterase/paraoxonase is a glycoprotein with a minimal molecular weight of about 43,000. It has up to three sugar chains per molecule, and carbohydrate represents about 15.8% of the total weight. The enzyme has an isoelectric point of 5.1. Its amino acid composition shows nothing unusual, except for a relatively high content of leucine. We conclude that human serum arylesterase and paraoxonase activities are catalyzed by a single enzyme, capable of hydrolyzing a broad spectrum of organophosphate substrates and a number of aromatic carboxylic acid esters. Studies on the genetically determined polymorphism responsible for two allozymic forms (A and B) of the esterase are described in the following paper.