Polygalacturonase, cellulase and invertase activities during cherimoya fruit ripening

The activities of the cell-wall degrading enzymes polygalacturonase (PG) and cellulase and of two forms of invertase were studied during the ripening of cherimoya fruit. All activities showed maxima coinciding with the climacteric maximum in respiration and the beginning of the most pronounced rise in ethylene production. The activities of PG and cellulase increased three-fold concomitantly with the highest drop in flesh firmness. After reaching maxima, the activity of the soluble invertase was almost stabilized, while that of the cell-wall bound form decreased to values similar to those of preclimacteric fruit. Since fructose and glucose concentrations increased continuously during ripening it is suggested that sucrose was hydrolyzed mainly by the soluble invertase. Cherimoya PG and cellulase had maximum activities at pH 5.0 and 5.5, respectively. PG activity was more thermostable than cellulase activity (complete inactivation by heating for 5 min at 75°C and 60°C, respectively). The activity of both invertases had a similar pH optimum (3.0-3.5), heat stability (complete inactivation by heating for 5 min at 55-60°C) and affinity for sucrose (K(m) of 2.6 and 3.8 mM).