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 α, β, and γ 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 α and γ was a weak inhibitor of chymotrypsin, whereas the dimer of protomer β did not inhibit the enzyme. Therefore TTI is presumed to be a mixture of heterogeneous and homogenous dimers whose properties reflect whose 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 α and γ, i.e., TTI-II is a heterogeneous dimer of protomers α and γ, and TTI-I and TTI-III are homogeneous dimers of protomers α and γ, 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.