A Proteolytic Enzyme of Serum: Characterization, Activation, and Reaction with Inhibitors

1. Fibrinolysin-activated lysin factor and chloroform-activated serum protease of serum and plasma are one and the same enzyme, differing only in their mode of activation. 2. The enzyme as it normally occurs in serum or plasma is not inactive because of combination with serum inhibitor. It is present as an inactive precursor or zymogen and may be activated from this state by streptococcal fibrinolysin. 3. The activation of serum protease by streptococcal fibrinolysin is a catalytic reaction, analogous to the kinase activation of trypsinogen by enterokinase. Treatment of serum or plasma with chloroform apparently results in removal of serum inhibitor which may allow autocatalytic activation of the serum protease. 4. The serum enzyme differs from trypsin in its pH of optimum activity, in its reactions with specific protease inhibitors, and in its action on casein. 5. A revised nomenclature for the serum enzyme system is suggested which more accurately describes its properties than the terms in current use.