Radiation damage to Bowman-Birk soybean proteinase inhibitor was studied in dilute aqueous solutions containing: (a) air, (b) N2 O plus KBr, and (c) N2 O plus KCNS. The degradation of tyrosines, monitored by changes in ultraviolet absorption and circular dichroism (CD) spectra of irradiated inhibitor, led to the loss of chymotrypsin-inhibitory activity without affecting antitrypsin activity. Of two tyrosyl residues in the inhibitor, Tyr 45, located next to the antichymotrypsin site, was shown to be essential to chymotrypsin-inhibitory activity. Radiation damage to Tyr 59 had no effect on either of the antiproteinase activities. The loss of trypsin-inhibitory activity paralleled decrease in disulfide CD. The breakage of disulfide bonds was confirmed by the p-chloromercuribenzoate assay for sulfhydryl groups. However, the exact role of disulfide bonds in antitryptic activity, other than the maintenance of conformational integrity, is not apparent. It seems more likely that the loss of trypsin-inhibitory activity is due to damage in other amino acid(s), although the degradation of tyrosine (2 residues) and histidine (1 residue) did not affect antitryptic activity.

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