Subterminal polygalacturonase, a nonmacerating enzyme, attacks pectate from the reducing end

Subterminal polygalacturonase from Aspergillus, which fails to macerate soft plant tissue in spite of a rapid action on pectate in vitro, was examined for its action at pH 3.5 on substrate (degree of polymerization 9-50) altered by the reduction of the reducing end to (3)H labeled l-galactonic acid, and the introduction of unsaturation in a portion of the nonreducing end groups. Endo-polygalacturonase from Saccharomyces fragilis was used as a control. The hydrolysis products were separated by gel filtration chromatography and the sugar residues, the tritium label, and the ultraviolet absorption (of the unsaturated groups) were measured. Endo-polygalacturonase gave equal production of the two end-labeled oligomers. Subterminal polygalacturonase rapidly produced a mixture of tritiated oligomers (mainly trimer, dimer, and tetramer), 2.5 times faster than it liberated unsaturated oligomers, and 3 times faster than it liberated unlabeled oligomers, showing that its action begins at the reducing end. The unsaturated pentamer and hexamer, which accumulated during the rapid phase of enzyme action, were subsequently hydrolyzed to the unsaturated tetramer, in accord with action from the reducing end.