Studies on an esterase derived from partially purified preparations of the first component of complement are described. The esterase hydrolyzed certain synthetic amino acid esters, among which N-acetyl-L-tyrosine ethyl ester was most susceptible. This was hydrolyzed maximally between pH 7.5 and 8.2, and at 41°C. The esterase could not be identified with other previously described hydrolytic enzymes. An esterase with similar properties could also be eluted from antigen-antibody aggregates which had been treated with serum.

Human serum contained a heat-labile inhibitor of the esterase which could not be identified with any of the known components of complement. The esterase was also inhibited by certain reducing agents.

The experiments described support the early hypothesis that complement exerts its action enzymatically, but the physiological role of the esterase derived from preparations of complement is not yet clear.

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