SUMMARY—Three polygalacturonic acid preparations with widely differing molecular weight distributions were obtained by controlled enzymatic hydrolysis of pectic acid. A study of the action of tomato polygalacturonase on the polygalacturonic acids and pectic acid revealed that the activity h dependent on the molecular size of the substrate. Pectic acid was hydrolyzed optimally at pH 5, with little activity below pH 4. Decreasing the molecular weight of the substrate resulted in a progressive shift of the pH optimum to the acid side. For the smallest substrate, the activity extended to below pH 2. Monovalent cations enhanced the activity at low pH, and this effect was also dependent on the molecular weight of the substrate. Below pH 4, pectic acid inhibited 70 of the hydrolysis of low molecular weight substrates by tomato polygalacturonase. The incomplete inhibition is attributed to the presence of a polygalacturonase isoenzyme which is not inhibited by high molecular weight polygalacturonic acid
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