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Schizolobium parahyba chymotrypsin inhibitor (SPCI) was completely purified as a single polypeptide chain with two disulfide bonds, by TCA precipitation and ion exchange chromatography. This purification method is faster and more efficient than that previously reported: SPCI is stable from pH 2 to 12 at 25 degrees C, and is highly specific for chymotrypsin at pH 7-12. It weakly inhibits elastase and has no significant inhibitory effect against trypsin and alpha-amylase. SPCI is a thermostable protein and resists thermolysin digestion up to 70 degrees C.

Schizolobium parahyba seed chymotrypsin inhibitor (SPC) is a protein with M r of 20 000 and four halfcystine residues and no free thiol group. SPC is stable at temperatures up to 75° at pH 7 but gradually loses activity when kept at 95° for 1 hr and total inactivation occurs after 5 hr. Amino acid analysis shows a high content of glycine, aspartate, glutamate and alanine residues. A pI of 4.52 predicted from the amino acid content agrees with experimental results. A stable binary complex with M r of 45 000, Ki = 5.85 × 10 −8 M and molar ratio of 1:1 is formed between SPC and chymotrypsin. The determined single N -terminal sequence of SPC shows homology with Kunitz type soybean trypsin inhibitors.