In VitroandIn SituProperties of Cell Wall Pectinmethylesterases From Mung Bean Hypocotyls

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Abstract
Pectinmethylesterases (EC 3.1.1.11) have been solubilized from young and mature tissues of mung bean hypocotyls. Whatever the plastic potential of the tissues, most of the pectinmethylesterase activity was located in the cell walls. Several active fractions were obtained after chromatography on CM Sépharose. Equilibrium sedimentation in an analytical ultracentrifuge indicated the MW of the isolated isoforms to be close to 75 000 whereas SDS-PAGE electrophoresis gave a MW around 32 000, suggesting the possibility of dimeric structures. Mung bean pectinmethylesterase (PME) showed cross reactivity with soybean antiserum. Experiments carried out with p-nitrophenylacetate and Citrus pectin revealed that PME and esterase activities might correspond to different isoforms. It was also noted that the stimulation induced by cations was stronger when the enzymes were bound to the cell walls. The high ionic sensitivity suggested that, in situ, the ionic environment regulates pectinmethylesterase activity principally by modifying the pectin molecules, which enhances the affinity of the enzymes for their substrate. These data indicate the importance of the calcium content of the cell walls and might explain the decrease in methylated pectins along the mung bean hypocotyl and, in turn, the loss of plasticity.

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