Titration and subunit localization of active center cysteine in fibrinoligase (thrombin-activated fibrin stabilizing factor)

Abstract Fibrinoligase activity, measured in a fully synthetic substrate system, was shown to be inhibited by prior incubation of the enzyme with iodoacetamide. Using 1- 14 C-iodoacetamide, the amount of incorporated radio-activity increased in proportion to the loss of enzyme activity. Labelling of the enzyme was totally dependent on the addition of calcium ions and, under the conditions described, occurred predominantly in the a ′ protomer. Acid hydrolysis of labelled fibrinoligase gave rise to S-carboxymethylcysteine as the only radioactive amino acid derivative.