Antithrombin from bony fish (Teleostei), represented by an ancient sal
monid, Atlantic salmon (Salmo salar L.), and a more evolved species fr
om the same family, rainbow trout (Oncorhychus mykiss Walbaum), functi
ons in vitro as does its human counterpart: it inactivates thrombin al
most instantaneously in the presence of heparin and only slowly when h
eparin is absent. The inhibitory activity of salmonid antithrombin tow
ards the homologous thrombin did not differ noticeably from its inacti
vating capacity in heterologous (teleost) systems, and enzyme-inhibito
r reactions between reagents from fish and man proceeded just as effic
iently. In all enzyme-inhibitor reactions with salmonid thrombin the i
nactivation by salmonid antithrombin or diluted fish plasma was maxima
l at pH 7.8 - 8.4. The inactivation was clearly dependent on heparin i
n all systems and maximal at concentrations between 1.5 and 6 U/ml. Wh
at particularly distinguishes the salmonid thrombin-antithrombin inter
action from the human one is that the former has to function over a wi
de range of temperatures. And the thrombin inactivating capacity of pu
rified antithrombin and diluted plasma in the presence of heparin was
indeed present at temperatures down to 3 degrees C, a capacity that hu
man antithrombin also has retained. Even more interesting was that the
teleost enzyme-inhibitor reaction was nearly independent of temperatu
re under the conditions studied.