An Apios americana trypsin inhibitor, AATI, was purified from Apios tubers by chromatography on DEAE Cellulofine A-500 and Sephadex G-50. The molecular mass of AATI was determined to be 6,437 Da by matrix-assisted laser desorption and ionization time-of-flight mass spectrometer (MALDI-TOF-MS). It showed strong inhibitory activity toward serine proteases, and the inhibition constants toward trypsin and chymotrypsin were 3.0 x 10(-9) M and 1.0 x 10(-6) M respectively. The inhibitory activity was not affected by heating at 80 degrees C for 2 h or by incubation at a wide range of pH values, suggesting that AATI has remarkable heat-stability and pH-stability. AATI cDNA consists of 552 nucleotides, and includes an open reading frame encoding a protein of 116 amino acids. The results of N-terminal amino acid sequencing of AATI and MALDI-TOF-MS analysis suggested that the deduced amino acid sequence had 50 and seven extra amino acids at the N- and C-termini respectively. Thus the mature AATI protein consists of 59 amino acid residues. Comparison of the amino acid sequence with those of the trypsin inhibitors from plants suggests that AATI belongs to the Bowman-Birk family and that it contains two possible reactive sites toward trypsin at Lys62 and Arg88.