Biosynthesis of water-soluble metabolites of UDP-D-galactose containing D-galactose by an enzymic preparation isolated from tissue culture of poplar (Populus alba L., var. pyramidalis)

Abstract

Biosynthesis of constituents of pectic polysaccharides containing D-galactose was investigated in poplar tissue culture in vitro. The particulate enzymic preparation isolated from the tissue culture of poplar catalyzed the transfer of D-[U-14C]-galactose unit form UDP-D-[U-14C]-galactose into various endogenous acceptors to form galactolipides, water-soluble metabolites and membrane-bound galactoproteins. The initial rate of formation of water-soluble metabolites depends on the substrate concentration and is time and amount of the enzymic protein directly proportional. Optimal formation of these metabolites proceeds at pH 7·6 and 40 °C in the presence of 0.1 mM ATP and 10 mM MgCl2. Long-term incubations afforded a mixture of three radioactive water-soluble metabolites: a neutral oligomer of Mr ~ 620 consisting at least of two β-D-[U-14C]-galactose units linked by (1 → 6) bonds and two oligomers of Mr ~ 1 400 and ≥ 2 950, respectively, containing (1 → 6) and (1 → 4)-linked D-[U-14C]-galactose units. Their relative proportion in the mixture changed in relation to incubation time and the substrate concentration. Structural features of these metabolites resembled those of D-galactans or L-rhamnogalacturonan side chains previously found in primary cell walls of some higher plants.