DIFFERENT FORMS OF TOMATO PECTINESTERASE HAVE DIFFERENT KINETIC-PROPERTIES

The kinetic properties of the three main forms (A-C) of pectinesterase (PE) found in pericarp of the tomato variety Ailsa Craig were investi gated. Differences in the effects of salt on the three forms were corr elated with the degree of charge carried by each form, as reflected in their isoelectric points (A: 8.9, B:9.7 and C:9.9). All three forms s howed Michaelis-Menten kinetics, with K-m varying with salt concentrat ion and degree of esterification (DE) of the substrate. PE-C generally exhibited lower values for K-m than PE-A, and was more effective at d e-esterifying pectin with low DE. A large difference was seen in the K -i for inhibition by polygalacturonate (A: 11.0 mM, B: 1.2 mM, C:0.7 m M). Calcium was particularly effective in activating PE-C. PE-C was co mpletely inactive once the pH was reduced to 5.0, while PE-A maintaine d more than 60% of maximum activity at this pH. These results suggest that the different kinetic properties of the three forms of PE reflect different physiological functions. PE-A should be able to continue wo rking during the processes of disruption and degradation that typify f ruit ripening, while the other forms (particularly PE-C) have their ac tivity under continuous fine control by local ionic and pH conditions in the wall, and are more likely candidates for a role in the modulati on of cell wall growth.