VARIOUS PECTIN METHYLTRANSFERASE ACTIVITIES WITH AFFINITY FOR LOW ANDHIGHLY METHYLATED PECTINS

The activity of pectin methyltransferases (PMT) from endomembranes of flax cells (Linum usitatissimum L.) was enhanced in the presence of ex ogenous pectins. The value of optimal pH increased from 5.5 to 7.0 wit h the degree of methylesterification (DE from 0.00 to 0.50) of pectins . We showed, using size exclusion chromatography, that methylesterific ation had principally occurred onto exogenous pectins. PMT activity, m easured in vitro at pH 7.0 and in the presence of highly methylated pe ctins, 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 low-me thylated pectins over the maturation phase. Two successive sucrose-gra dient centrifugations led to the fractionation of low-density membrane s (density 1.08) with PMT activity only detected at pH 5.5 and in the presence of low-methylated pectins (DE 0.10), On the other hand, membr anes of density 1.12-1.14 were enriched in PMT with a maximum of activ ity that happened at pH 7.0 and in the presence of highly methylated p ectins (DE 0.50). These experiments indicated two types of pectin meth yltransferase activities, However, their apparent K-m(s) for the donor of methyl, S-adenosyl methionine (about 20 mu M), and for the pectic substrate (1 mM galacturonic acid or 0.25 mg ml(-1)) were similar.