Cystatin-like domain of mouse countertrypin, a member of mammalian fetuin family, is responsible for the inhibition of trypsin. Evidence from site-directed mutagenesis

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Abstract
Members of mammalian fetuin family, such as human α2HS glycoprotein and bovine fetuin, consist of three domains, two tandemly arranged cystatin‐like domains and an unrelated domain, but they have no inhibitory activity against cysteine proteinases. We found that countertrypin, a novel trypsin inhibitor, is mouse counterpart of human α2HS glycoprotein, and that human α2HS glycoprotein and bovine fetuin which were prepared without use of ethanol are capable of inhibiting trypsin (Yamamoto, K. and Sinohara, H. (1993) J. Biol. Chem, 268, 17750‐17753). In the present study, cDNA encoding countertrypin was isolated and sequenced, and evidence is presented, based on the site‐directed mutagenesis, that lysine‐231 in the second cystatin domain is the P1 site for trypsin inhibition.

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