Strong activity of acid-stable trypsin inhibitor in bovine thrombin for clinical use.

Strong activity of acid-stable trypsin inhibitor (ASTI) was confirmed in some clinical thrombin preparations. Thrombin preparations of human plasma origin had no detectable ASTI activity, whereas some preparations of bovine plasma origin revealed more than 5,000 U/vial (5,000 thrombin units), indicating a higher content of ASTI than of thrombin in terms of protein concentration. Contamination by other biologically active substances was also suggested by variations in amidolytic activity with several synthetic substrates (S-2238, S-2251, S-2444, S-2266 and Bz-L-Arg-pNA). On isoelectric focussing, the ASTI activities migrated in acidic positions with pI values of 3.9, 4.5, 5.0, 5.9 and 6.5, respectively. They were almost parallel to the thrombin Bz-L-Arg-pNA hydrolytic activity, and differed from that of the purified thrombin preparation (pI = 7.0). By gel filtration on Sephadex G-100, the molecular weights of the inhibitors as calculated using standard proteins were 140,000 (main), 70,000 and less than 10,000 (minor), respectively. An immunological difference between the main inhibitor (pI = 3.9, mol wt 140,000) and previously reported plasma ASTI was also confirmed with goat anti-UTI serum by the double immunodiffusion and ELISA methods. The inhibitor exerted a strong inhibitory effect not only on trypsin and chymotrypsin, but also on non-plasmic fibrinolysis with human leukocyte elastase, and to a lesser extent on the blood coagulation system (lengthening of APTT and PT). Clearly, when using thrombin preparations and analyzing the data obtained after their administration, the effects of this and other contaminant biologically active substances must be taken into account.