Subunit compositions of trypsin inhibitors from tubers of taro, Colocasia antiquorum var. nymphaifolia?

Several trypsin inhibitors with different mobilities on polyacrylamide gel electrophoresis occur in the tubers of taro (Colocasia antiquorum), and they each have a dimeric molecular weight of 40,000. Of all the constituent subunits, molecular weight 20,000, of the taro trypsin inhibitor (TTI), three major subunit components were separated by chromatography on SP-Sephadex C-25 in 8 M urea, and they were named protomers alpha, beta, and gamma in the order of their elution from the SP-Sephadex column. After removal or dilution of the urea, the three protomers could be either reassociated individually or hybridized with each other to form dimeric inhibitors. All of the reassociated dimers were powerful inhibitors of trypsin. Among them, each dimer derived from protomers alpha and gamma was a weak inhibitor of chymotrypsin, whereas the dimer of protomer beta did not inhibit the enzyme. Therefore TTI is presumed to be a mixture of heterogeneous and homogenous dimers whose properties reflect those of their constituent protomers. It was also proved that the major three trypsin inhibitors (TTI-I, TTI-II, and TTI-III) previously isolated from taro tubers are composed of protomers alpha and gamma, i.e., TTI-II is a heterogeneous dimer of protomers alpha and gamma, and TTI-I and TTI-III are homogeneous dimers of protomers alpha and gamma, respectively. The molecular weight of a trypsin-TTI complex saturated with trypsin was found to be 79,000, suggesting the formation of a tetrameric complex.