A water-soluble substance in grape leaves which inhibits both purified polygalacturonase (PG) and pectinase from mold-laden cucumber flowers is reported. 2. Of the six grape varieties tested, the leaves from the Scuppernong variety of the Muscadine group (Vitus rotundifolia Michx.) contained the highest inhibitor content. 3. The inhibiting substance was stable to heat, non-dialyzable through cellophane membrane against running tap water and distilled water, and could not be completely precipitated with acetone or concentrated ammonium sulfate. 4. The reduction in pectinase activity obtained was directly related to the inhibitor concentration used. The reaction between polygalacturonase, the substrate, and the inhibitor (acetone-powder preparation) was that of competitive inhibition.
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Bell et al. (1958) studied this question.