Automated measurement of protease-inhibitor capacity of serum, by use of chymotrypsin and casein.

Serum protease inhibitor activity was assessed from the ability of the serum to inhibit casein hydrolysis by ct-chymotrypsin (EC 3.4.4.5). Unmodified AutoAnalyzer equipment was used in conjunction with a fluorimetric method for dialyzable tyrosyl peptides. Serum, diluted with tris(hydroxymethyl) aminomethane buffer, was added to a chymotrypsin solution to inhibit its activity. For 72 sera, the automated method was compared with a manual determination of trypsin inhibitor capacity. Results by the two methods correlate well, supporting previous evidence that trypsinand chymotrypsin-inhibitor capacities of serum are related. The present automated method has several advantages that make it useful for clinical studies.