Characteristics of the antitrypsin activity of human serum

The antitrypsin activity of human serum was studied as a function of pH, preincubation of serum with trypsin, and the concentrations of substrate, trypsin, and serum. Maximal activity was observed between pH 8.0 and 8.3. Activity was enhanced by preincubation of serum with trypsin, and reduced by high concentrations of trypsin and substrate. Percentage inhibition was a sigmoid function of log serum concentration. Theoretically, this relationship is expected of a reversible enzyme inhibitor; reversibility was demonstrated by dilution of enzyme serum mixtures. Practically, this relationship limits accurate antitryptic activity assay to the 25-75% inhibition range.