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The activity of pectin methyltransferases (PMT) from endomembranes of flax cells (Linum usitatissimum L.) was enhanced in the presence of exogenous pectins. The value of optimal pH increased from 5.5 to 7.0 with the degree of methylesterification (DE from 0.00 to 0.50) of pectins. We showed, using size exclusion chromatography, that methylesterification had principally occurred onto exogenous pectins. PMT activity, measured in vitro at pH 7.0 and in the presence of highly methylated pectins, was maximum when tested during the fast growth-phase of cells. In contrast, a major peak occurred at pH 5.5 in the presence of lowmethylated pectins over the maturation phase. Two successive sucrose-gradient centrifugations led to the fractionation of low-density membranes (density 1.08) with PMT activity only detected at pH 5.5 and in the presence of lowmethylated pectins (DE 0.10). On the other hand, membranes of density 1.12-1.14 were enriched in PMT with a maximum of activity that happened at pH 7.0 and in the presence of highly methylated pectins (DE 0.50). These experiments indicated two types of pectin methyltransferase activities. However, their apparent A"m(s) for the donor of methyl, S-adenosyl methionine (about 20 //M), and for the pectic substrate (1 mM galacturonic acid or 0.25 mg ml" 1 ) were similar.

Various Pectin Methyltransferase Activities with Affinity for Low and Highly Methylated Pectins