Pectin methylesterase (PME), polygalacturonase (PG), xylanase, cellulase, and proteinase activty were determined and related to respiration, ethylene evolution, and changes in skin color of papaya (Caricapapaya L.) fruit from harvest through to the start of fruit breakdown. PME gradually increased from the start of the climacteric rise reaching a peak 2 days after the respiratory peak. PG and xylanase were not detectable in the preclimacteric stage but increased during the climacteric: during the post climacteric stage, the PG declined to a level one-quarter of peak activity with xylanase activity returnig to zero. CeHlulase activity gradually increased 3-fold after harvest to peak at the same time as PME, 2 days after the edible stage. Proteinase declined throughout the climacteric and postclimacteric phases. A close relationship exists between PG and xylanase and the rise in respiration, ethylene evolution, and softening. Cultivar differences in postclimacteric levels of enzymic activity were not detected.An inhibitor of celHulase activity was detected in preclimacteric fruit. The inhibitor was not benzyl isothiocyanate (BITC). BITC did inhibit PG activity, though no inhibitor of PG activity was detected in preclimacteric homogenates when BITC was highest. The results indicate that inhibitors did not play a direct role in controlling wail softening.The postharvest papaya fruit ripening involves softening and the production of sugars and flavor constituents. There is a concomitant evolution of ethylene and an increased respiration rate (2, 17). The softening of the mesocarp and endocarp is due to the activity ofcell wall-degrading enzymes, not starch degradation, as the fruit has no starch during ontogeny (8, 10).Chan et al. (10) studied the relationship between fruit ripening, skin color, and the level of PG.2 PG increased during fruit ripening and was greatest near the endocarp. The enzyme has both exoand endopolygalacturonase activity (9). The role of other enzymes in wall degradation is unclear, and the relationship between walldegrading enzymic activity, softening, respiration, and ethylene production has not been determined. There is variation between papaya cultivars in the rate of softening and particularly the rate at which the flesh loses all texture and becomes water soaked. Inasmuch as there is considerable variation in the time of ripening of individual fruit, we report here the changes in wall-degrading enzymes in relation to respiration and ethylene production in a single fruit. Three cultivars are compared as to the levels of postclimacteric wall-degrading enzymes. The information is cru- cial to the understanding of ripening and for selecting cultivars with desirable postharvest ripening characteristics.MATERIALS AND METHODS Papaya (Carica papaya L. cv Sunrise) grown at the Poamoho Experimental Station on the island of Oahu were used. Cultivar Kapoho Solo was obtained from the island of Hawaii and X-77 from the island of Oahu. Fruit was harvested at the mature green stage. Fruit was hot...
Mesocarp softening during papaya (Carica papaya L.) ripening was impaired by heating at 42C for 30 min followed by 49C for 70 min, with areas of the flesh failing to soften. Disruption of the softening process varied with stage of ripeness and harvest date. The respiratory climacteric and ethylene production were higher and occurred 2 days sooner in the injured fruit than in the noninjured fruit that had been exposed to 49C for only 30 min. Skin degreening and internal carotenoid synthesis were unaffected by the heat treatments. Exposure of ripening fruit to either 42C for 4 hr or 38 to 42C for 1 hr followed by 3 hr at 22C resulted in the development of thermotolerance to exposure to the otherwise injurious heat treatment of 49C for 70 min. Four stainable polypeptide bands increased and seven declined in single-dimensional acrylamide gels following incubation of fruit at the nondamaging temperature of 38C for 2 hr. Three polypeptides showed marked increases when polysomal RNA was translated. These polypeptides had apparent molecular weights of 17, 18, and 70 kDa. Proteins with molecular weights of 46, 54, and 63 kDa had slight increases after heat treatment. The levels of these polypeptides peaked 2 hr after heat treatment and declined within 24 hr. The amount of these polypeptides in the unheated control varied with the batch of fruit. The concentration of three translated polypeptides, with apparent molecular weights of 26, 37, and 46 kDa, declined. Other polypeptides continued to be translated during and after holding papayas for 2 hr at 38C.
Papaya (Carica papaya L.) softening during fruit ripening is correlated with the activities of an endoxylanase (EC 3.2.1.8). A 32.5-kDa xylanase (CpaEXY1) from ripening fruit mesocarp was purified 45 871-fold on enzymatic activity and to homogeneity by SDS electrophoresis. The enzyme had endo- and not exo-xylanase activity, a pH optimum of 5–7 and was inhibited by Ca2+, Co2+, and Zn2+. Degenerate primers were constructed from five peptides obtained from the purified enzyme, and a full-length cDNA clone (AY138968) was isolated from a library constructed from ripening mesocarp. CpaEXY1 coded for a 64.96-kDa protein that had up to 61% identity with the 12 predicted Arabidopsis Family 10 endoxylanase-like sequences and 40% to the barley aleurone xylanase. The peptide sequences, obtained from the trypsin-digested purified protein, were all found between amino acid 267�and 426 out of the predicted 584 amino acids. The N-terminal 27 amino acids were hydrophobic and formed a predicted secretory signal peptide. A predicted carbohydrate-binding module was located between amino acids 60�and 182, distinct from the C-terminal endoxylanase catalytic center. CpaEXY1 was developmentally expressed during fruit ripening and the expression correlated with the variation in softening patterns of different varieties. The findings are consistent with the hypothesis that CpaEXY1 was expressed during fruit ripening; the expression was correlated with softening and was modified by post-translational proteolysis. This modification may take place in the cell wall, and regulate enzyme activity and cell-wall-microdomain-specific hydrolysis.
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