Pectic methyl and nonmethyl esters in potato cell walls

Because pectins are released from potatoes and other plants under conditions that cleave ester linkages, it has been suggested that there are other galaturonoyl ester cross-links between pectin chains in addition to the known non-cross-linking methyl esters. A microscale titration method and a copper binding method were developed for the measurement of total polymer carboxyl (essentially pectic) ester content in potato cell walls. Relative to the uronic acid content of the cell walls, the degree of total esterification was 57-58%. Comparison with levels of methanol released on ester hydrolysis allowed nonmethyl uronoyl esters to be estimated to be 14-15% relative to total uronic acid. The possibility of nonmethyl-esterified linkages being formed in potato cell walls by a side-reaction catalyzed by pectin methyl esterase (PME) was investigated, but no increase in nonmethyl-esterified pectin was observed under conditions where pectin was being effectively de-esterified by endogenous PME activity.