Exopolygalacturonase protein accumulates late in peach fruit ripening

Two exo‐acting polygalacturonase enzymes (exoPG, EC 3.2.1.67) increase in activity as peach ( Prunu persica L. Batsch cvs Coronet and Flavorcrest) fruits ripen. By examining populations of fruit, we show that the increase in activity occurs late in ripening when the fruit are very soft (below 2 kgf). The more abundant form of the enzyme, exoPG 2, was extensively purified and analyzed for its amino acid content and N‐terminal amino acid sequence. ExoPG 2 is a polypeptide of M, 66 000 and has a substantial excess of basic over acidic amino acids. Polyclonal antisera to exoPG 2 were raised in mice. The antisera inhibited the enzyme activity and recognized a M r 66 000 polypeptide in Western blots. Western blot analyses of extracts of fruit ranked for softness revealed a M r 66 000 polypeptide only in the softest fruit (less than 2.5 kgf). We conclude that the increased in exopolygalacturonase activity that occurs in very soft fruit is due to an increase in the amount of enzyme protein.