Study on the Barley and Malt Polyphenoloxidase. II. Characterization of Barley and Steeped Barley Polyphenoloxidase. Van Huynh, N., and Jerumanis, J., Section de Brasserie, Universite de Louvain, B-1348, Louvain-la-Neuve, Belgium. J. Am. Soc. Brew. Chem. 36:0081, 1978.

The properties of the lower mol wt polyphenoloxidase (L-enzyme) isolated from barley and steeped barley were studied. The mol wt measured by sodium dodecyl sulfate electrophoresis was 39,000. The enzyme showed an absorption at 280 nm (A 1%/1 cm = 4.3) with a shoulder at 290 nm. The optimum pH range was at 6.5-7.5. The enzyme was thermostable with a half-life equal to 1 hr at 65° C. An apparent activation energy of 4.2 kcal/mol was calculated for the catechol oxidation between 20 and 40° C. Kinetic constants were determined for a number of substrates, including the natural polyphenols of barley, and inhibitors. After steeping, the enzyme was more active due to a high increase in the maximum velocity (V_M) and a weak decrease in the Michaelis constant (K_M) for catechol and p-cresol. The best substrates were (+)- and (-)-catechins. The enzyme was inactive towards (-)-epicatechin. The enzyme was inhibited competitively by 4-nitrocatechol and non-competitively by sodium cyanide. The data reported here are fully consistent with the description of the barley enzyme as a monophenol, dihydroxyphenylalanine:oxygen oxidoreductase, EC 1. 14. 18. 1.

Keywords: Enzyme kinetics, Inhibitor, Molecular weight, Substrate specificity.