Sorghum proteinase inhibitors. 2. Mode of interaction with serine proteinases

Investigations have been carried out on the complex formed between sorghum Inhibitor III and alpha-chymotrypsin by physico-chemical methods. An apparent dissociation constant (Ki) of 4.0 X 10(-8) M has been calculated for the complex. This enzyme-inhibitor complex was isolated by gel filtration on Sephadex G-75 and a molecular weight of 48,000 was estimated for the complex. The formation of the complex was accompanied by spectral changes in the 270--300 nm region of the spectrum. Preliminary evidence suggests that the sorghum Inhibitor III is structurally altered when it is incubated with alpha-chymotrypsin. Catalytically inactive derivative of alpha-chymotrypsin and trypsin, as well as their zymogens, did not interfere with the activity of the sorghum inhibitor towards the native enzymes. Sorghum Inhibitor I was shown to be a 'double-headed' inhibitor since it inhibits both trypsin and alpha-chymotrypsin independently.