The catalytic capacity of tomato polygalacturonase (PG) toward soluble pectic polymers is in excess of activity expressed in vivo; however, in vitro assays of PG have traditionally been performed under conditions (pH 4.0 to 4.5, 150 mM NaCl) that likely do not reflect the apoplastic environment of ripening tomato fruit. In this study, hydrolysis of pectin by purified tomato PG (isozyme 2) was examined in response to K+ (the predominate apoplastic cation) and over the pH range from 3.0 to 6.0. In the presence of K+, PG activity toward polygalacturonic acid measured reductometrically increased nearly 3.5-fold from pH 4.0 to pH 5.5. In the presence of Na+, activity decreased 90% over the same pH range.
PG-mediated degradation of cell wall from mature-green fruit showed divergent hydrolytic patterns in response to pH and K+. At pH 4.5 in the presence of K+ (as KCl), catalysis resulted in both solubilization and extensive depolymerization of cell wall pectin, with oligomers accounting for a significant portion of the hydrolysis products. At pH 5.5, the total quantity of wall pectin released in response to PG2 was similar to that at pH 4.5; however, oligomer production was strongly suppressed at the higher pH. At pH values favoring extensive depolymerization, low mol mass products were produced at 5 mM K+ and increased to a maximum at 100 mM K+. At higher pH, hydrolysis patterns were not affected by [K+]. pH and ionic effects may contribute to the distinctive patterns of pectin hydrolysis observed for different fruits.