Optimization of enzyme-based assays in coagulation testing

Optimized assays for antithrombin III and plasminogen have been developed based on a study of the kinetic parameters Km and Kcat for four commercially available substrates: the p-nitroanilide derivatives of D-Phe-pipecolyl-Arg (S-2238), and toluenesulfonyl-Gly-Pro-Arg (Chromozym TH), which are thrombin substrates; D-Val-Leu-Lys (S-2251), a plasminogen/streptokinase substrate; and alpha-N-carbobenzoxy-L-lysine thiobenzyl ester, a substrate for both enzymes. We used a centrifugal analyzer system for rapid data acquisition and interactive analysis. Optimized conditions for assay of a particular enzyme are not constant for different substrates in the same buffering agent. For example, in 1,4-piperazine diethanesulfonic acid buffer at 37 degrees C, thrombin-catalyzed hydrolysis of Chromozym TH is optimal at 125 mmol/L buffer, 100 mmol/L NaCl, and pH 8.2, whereas substitution of S-2238, also a tripeptide p-nitroanilide, yields optimal hydrolysis at 85 mmol/L buffer, 300 mmol/L NaCl, and pH 7.2. We conclude that optimized assay conditions are best obtained by an extensive survey of available buffers and a detailed investigation of the effects of variation in pH and in the concentrations of the buffer and auxiliary reagents through use of both one-factor-at-a-time and multivariate response surface experimentation.