PSEUDO-CHOLINESTERASE CONCENTRATION 311 3. The pseudo-cholinesterase concentrations have been expressed on the basis ofenzyme activity under standard conditions. The concentration of pseudocholinesterase in the particular samples of full serumused were as follows: horse serum 8 8 x 10-8 M; human serum, 7*9 x 10-8M; female mouse serum, 5-8 x 10-8M; dog serum, 4-6 x 10-8M; female rat serum, 17 x 108M. 4. The pseudo-cholinesterases from mouse, rat, horse, human and dog serum form a series in which the ratio of pseudo-cholinesterase activity towards 0-06M-acetylcholine over that towards 0-006Mbenzoylcholine diiminishes progressively from 8-2 to 1-6. In this same series, the absolute activity of the pseudo-cholinesterases towards acetylcholine becomes progressively smaller. Other characteristics of the pseudo-cholinesterases also vary considerably from one type of pseudo-cholinesterase to another, but do not show any correlation to the above series. 5. The significance of these results has been discussed with respect to the concept of the pseudocholinesterases as a special group of closely related enzymes. The author is indebted to Dr B. Mendel for his valuable criticisms and suggestions, to Miss M. de Jonge and Miss E.

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