IDENTIFICATION OF BOVINE STEFIN-A, A NOVEL PROTEIN INHIBITOR OF CYSTEINE PROTEINASES

For the first time, three different stefins, A, B and C, have been iso lated from a single species, The complete amino acid sequence of bovin e stefin A was determined, The inhibitor, with a calculated M(r) of 11 ,123, consists of 98 amino acid residues, Although it exhibits conside rable similarity to human and rat stefin A, some significant differenc es in inhibition kinetics were found, Bovine stefin A bound tightly an d rapidly to cathepsin L (k(ass) = 9.6 . 10(6) M(-1). s(-1), K-i = 29 pM), The binding to cathepsin H was also rapid (k(ass) = 2.1 . 10(6) M (-1). s(-1)), but weaker (K-i = 0.4 nM) due to a higher dissociation r ate constant, In contrast, the binding to cathepsin B was much slower (k(ass) = 1.4 . 10(5) M(-1). s(-1)),but still tight (K-i = 1.9 nM).