Partial Purification and Characterization of Two Barley Fractions that Inhibit Malt Endoproteinases¹

Abstract

The endoproteinase enzymes of malt are rate limiting for protein breakdown, so their activities determine the final amino acid-peptide-protein ratios of worts and of beers prepared from them. These ratios strongly affect many aspects of brewing quality. Proteinase inhibitors present in ungerminated barley can inactivate the enzymes of green malt, thus affecting beer quality. We have begun purifying and characterizing the protease inhibitors from barley. Two inhibitory fractions were isolated and studied. One, inhibitor A (I_A), is apparently a low molecular weight protein(s); the other, inhibitor B (I_B), is even smaller (MW 100–500). I_A has been purified nearly to homogeneity by open column and high-performance liquid chromatography, while I_B is still relatively impure. A portion of I_B was soluble in n-butanol and dimethyl sulfoxide, but it was insoluble in eight other organic solvents tested. Both inhibitors were heat stable and inactivated at least 75% of the endoproteinase activity present in a crude green malt extract.

• Hordeum vulgare
• Endoproteinase inhibitors