The Complete Amino Acid Sequence of a Trypsin Inhibitor from Bauhinia variegata var. candida Seeds

Trypsin inhibitors of two varieties of Bauhinia variegata seeds have been isolated and characterized. Bauhinia variegata candida trypsin inhibitor (BvcTI) and B. variegata lilac trypsin inhibitor (BvlTI) are proteins with Mr of about 20,000 without free sulfhydryl groups. Amino acid analysis shows a high content of aspartic acid, glutamic acid, serine, and glycine, and a low content of histidine, tyrosine, methionine, and lysine in both inhibitors. Isoelectric focusing for both varieties detected three isoforms (pI 4.85, 5.00, and 5.15), which were resolved by HPLC procedure. The trypsin inhibitors show Ki values of 6.9 and 1.2 nM for BvcTI and BvlTI, respectively. The N-terminal sequences of the three trypsin inhibitor isoforms from both varieties of Bauhinia variegata and the complete amino acid sequence of B. variegata var. candida L. trypsin inhibitor isoform 3 (BvcTI-3) are presented. The sequences have been determined by automated Edman degradation of the reduced and carboxymethylated proteins of the peptides resulting from Staphylococcus aureus protease and trypsin digestion. BvcTI-3 is composed of 167 residues and has a calculated molecular mass of 18,529. Homology studies with other trypsin inhibitors show that BvcTI-3 belongs to the Kunitz family. The putative active site encompasses Arg (63)–Ile (64).