Protein Inhibitors of Cysteine Proteinases. III. Amino-Acid Sequence of Cystatin from Chicken Egg White

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Abstract
Cystatin, the protein inhibitor of cysteine proteinases from chick egg white was purified by a new method. The 2 major forms with pI [isoelectric point] 6.5 (peak I) and 5.6 (peak II) were separated. Molecular masses of both forms are .apprx. 12,700 Da [daltons] as determined by gel chromatography; form A from peak I has a molecular mass of 12,191 Da as calculated from its amino-acid sequence. The complete amino-acid sequence of form A was determined by automated solid-phase Edman degradation of the whole inhibitor and its cyanogen bromide fragments. It contains 108 amino-acid residues. Form B from peak II represents an elongation of form A by 8 amino-acid residues at the N-terminus. Cystatin contains 4 cysteine residues, presumably forming 2 S-S bridges. Comparison of the amino-acid sequences and near UV circular dichroism spectra of stefin, the cysteine proteinase inhibitor from human granulocytes, and cystatin shows that the 2 proteins are entirely different. According to the primary structures, probably neither proteinase inhibitor is involved in a thiol-disulfide exchange mechanism in the interaction with its target enzyme.