Accessibility and structural function of particular amino acid residues of soy bean trypsin inhibition

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About 10 of the 11 lysine groups of soy bean trypsin inhibitor (STI) are accessible to reaction with methyl isourea. The guanidinated product has the same trypsin-inhibiting capacity as the original, although some activity is lost by a urea denaturation-renaturation cycle. From perturbation spectra the tryptophan groups of STI behave formally as if 67% exposed to solvent at pH 7. Only one tryptophan group is oxidized by N-bromosuccinimide in water at pH 4–6, with a minor loss of activity. Most of the residual activity survives a urea denaturation-renaturation cycle. Three tryptophans are oxidized for denatured inhibitor in 10 m urea, with nearly complete loss of activity. Two tryptophans are oxidized by H 2O 2 at pH 8.6 with complete inactivation. Two tyrosine groups are iodinated at, pH 8–9, with only a minor loss of activity. Most of the residual activity remains after a urea denaturation-renaturation cycle. Iodination of denatured inhibitor in 10 m urea results in the reaction of all four tyrosines, with total loss of activity. Both histidine groups react with diazonium-1-H-tetrazole. This, together with the kinetics of imidazole-catalyzed hydrolysis of p-nitrophenol acetate, suggests that both histidines are exposed to solvent.