When various rat tissues are incubated in homologous serum, a factor chemotactic in vitro for neutrophils is generated. The amount of chemotactic activity is a function of duration of incubation and the quantity of heart tissue or serum employed. Addition of trypsin inhibitor or antibody to the third component of complement (C3) precludes generation of chemotactic activity. In addition, antibody to C3 ablates chemotactic activity even after its formation.

Purified human C3 (ß1C-globulin) effectively substitutes for serum in the generation of chemotactic activity by heart tissue. The active product, as determined by gel filtration or by ultracentrifugal analysis in a sucrose density gradient, appears to be a cleavage product of C3 with a molecular weight of approximately 14,000. In addition, a larger C3 fragmentation product varying in molecular weight, depending upon experimental conditions, is also found.

The protease in rat heart tissue capable of cleaving C3 into chemotactic fragments is a serine esterase with trypsin-like properties and can be inhibited by organophophorous compounds or trypsin inhibitors. The use of amino acid esters in the manner of competitive substrate inhibition confirms the trypsin-like nature of the protease.

The presence of a protease in heart, and presumably in other normal tissues, capable of fragmenting C3 into factors with chemotactic activities may explain the development of the acute inflammatory response when tissues are non-specifically injured. If true, this would reinforce the role of the complement system in the mediation of nonimmunologically induced inflammation.

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