It has been found that under a wide range of physico-chemical conditions a positive correlation exists between the rate of disappearance of hemolytically active, partially purified first component of human complement and the rate of activation of an esterase hydrolyzing N-acetyl-L-tyrosine ethyl ester. Both reactions follow the kinetic equation for second order autocatalysis, with an apparent energy of activation of 31,000 calories per mol. They occur optimally at pH 7.3–7.7 and are inhibited by ionic strengths greater than 0.15, by 5 x 105 M ethylenediaminetetraacetic acid, and by a heat-labile serum inhibitor which appears unrelated to any component of complement. The activation of first component to esterase resembles closely the activation of trypsinogen to trypsin. Partially purified first component, containing plasminogen, may also be activated to esterase by addition of streptokinase.

The significance of these data with respect to the postulated existence of first component as a proesterase and its possible role in complement-"fixation" is discussed.

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