Investigations on the functional role of rat liver carboxylesterase isoenzymes.

We have compared the substrate specificities of two isoenzymes of rat liver esterase. Isoenzyme E1 which we isolated earlier, and isoenzyme EA, the isolation of which is reported here, have very similar molecular weights, subunit weights, and isoelectric points. In contrast, the two enzymes have distinctly different substrate optima when tested with a series of esterase substrates. Our finding poses the question, as to whether the rather unspecific in vivo behaviour of liver carboxylesterase could be explained by an ensemble of isoenzymes with overlapping substrate specificitites and mutually complementary substrate optima. The biological function of esterase isoenzymes appears to represent an extension of the range of hydrolytic competence of a single enzyme.