Characterization of the Isoenzymes of Pig‐Liver Esterase 2. Kinetic Studies

The kinetic properties of two of the partially separated isoenzymes I and V of pig liver esterase were studied. The cholinesterase‐like isoenzyme I hydrolyses butyrylcholine as well as various other esters and aromatic amides. This isoenzyme is sensitive to 0.01 mM physostigmine and to fluoride. The second type (isoenzyme V) has the features of the so‐called aliesterase: it acts preferentially on short‐chain aliphatic esters, does not hydrolyse butyrylcholine and has only low activity towards amides. Further differences exist with regard to sensitivity towards organophosphorous inhibitors, to the influence of organic solvents and to the pH optimum. Transacylation reactions with methanol as an acceptor are mainly catalyzed by the isoenzyme of the aliesterase type. The esterase forms III and IV, which are located between isoenzymes I and V on the isoelectric focussing column, show kinetic features similar to those of a mixture of I and V. A stepwise increase or decrease, respectively, of certain kinetic properties, e.g. specific activities, is observed for the sequence I, III, IV, V. A quantitative comparison of the kinetic properties supports our proposed subunit model [1], according to which the main components of these four esterase fractions are the trimers γγγ, αγγ, ααγ and ααα. These results offer explanations for many of the very complex and often controversial results formerly obtained with the heterogeneous pig liver esterase.