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Human placenta is surprisingly rich in post-proline dipeptidyl peptidase activity. Among various cell fractions, microsomes have the highest specific activity. A homogeneous enzyme preparation is obtained in a six-step purification procedure. The final preparation appears homogeneous upon dodecyl sulfate electrophoresis, but analytical isoelectric focussing reveals various active bands with isoelectric points in the range of pH 3 -4. The enzyme is a glycoprotein containing about 30 carbohydrate. Treatment with neuraminidase lowers the isoelectric points but does not reduce the heterogeneity of the band pattern. The subunit molecular weight is 120000 as estimated by dodecyl sulfate electrophoresis, whereas M, of the native enzyme is > 200000, as can be concluded from gel filtration experiments.The purified dipeptidyl peptidase cleaves various synthetic and natural peptides, including substance P, kentsin, casomorphin and a synthetic renin inhibitor. In general, the specificity of the placenta peptidase is similar to that of post-proline dipeptidyl peptidase from other sources. Phenylalanylprolyl-P-naphthylamide (K, = 0.02 mM,

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 I11 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, 111, IV, V. A quantitative comparison of the kinetic properties supports our proposed subunit model [l], according to which the main components of these four esterase fractions are the trimers y y y , ayy, m y , and aaa.These results offer explanations for many of the very complex and often controversial results formerly obtained with the heterogeneous pig liver esterase.Kinetic studies on pig liver esterase have been carried out extensively in many laboratories [2 -71.

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