Establishment of a Standard Assay Method for Human Thrombomodulin and Determination of the Activity of the Japanese Reference Standard

Abstract

This study was undertaken to establish a standard method for determination of the activity of human (h) thrombomodulin (TM). The reactions mainly consisted of formation of a h-TM and h-thrombin complex, activation of h-protein C by the complex and digestion of substrate by activated h-protein C. Linear time-dependent formation of p -nitroaniline from the substrate, S-2366, was observed up to 12min during measurement of the activity of urinary h-TM (uh-TM) reference material by the standard method. Therefore, 10minutes was established as the reaction time in the standard method. In the standard method, we defined the activity of h-TM forming 0·1μ mol of p -nitroaniline per min in the reaction as 1 JRS Unit. Recombinant h-TM (rh-TM) and uh-TM reference material gave rectilinear dose-response curves within a certain range of specific activities by their original methods in the standard method. The validity of the standard method was assessed based on the coefficients of variation (CV) obtained in the various measurements of h-TM. Intra-batch precision (CV) of h-thrombin and h-protein C was 2·90% and 6·57%, respectively, in the measurement of uh-TM activity. The intra-sample, inter-day, and inter-laboratory precision (CV) was 1·30%, 1·63% and 5·02%, respectively, in the measurement of the first Japanese reference standard for h-TM coded TJRS1. In these assays, the activity of the first Japanese standard was also determined and found to be 205 JRS units per ampoule. When the stability of the Japanese reference standard was assessed by measuring of the standard stored under various thermal conditions, the predicted loss of activity assuming monomolecular degradation according to the Arrhenius equation was less than 3·0% during 103 years at −20°C. These results indicate that the standard method is appropriate for determination of the activity of h-TM and that the Japanese reference standard for h-TM, whose activity was determined here, can be stored at −20°C for long periods without loss of activity.