GLUCOMANNAN SYNTHESIS IN PEA EPICOTYLS - THE MANNOSE AND GLUCOSE TRANSFERASES

Membrane fractions and digitonin-solubilized enzymes prepared from ste m segments isolated from the third internode of etiolated pea seedling s (Pisum sativum L. cv. Alaska) catalyzed the synthesis of a beta-1,4- [C-14]mannan mannan from GDP D-[U-C-14]-mannose, a mixed beta-1,3 and beta-1,4-[C-14]glucan from GDP-D-[U-C-14]-glucose and a beta-1,4-[C-14 ]-glucomannan from both GDP-D-[U-C-14]mannose and GDP D-[U-C-14]glucos e. The kinetics of the membrane-bound and soluble mannan and glucan sy nthases were determined. The effects of ions, chelators, inhibitors of lipid-linked saccharides, polyamines, polyols, nucleotides, nucleosid e-diphosphate sugars, acetyl-CoA, group-specific chemical probes, phos pholipases and detergents on the membrane-bound mannan and glucan synt hases were investigated. The beta-glucan synthase had different proper ties from other preparations which bring about the synthesis of beta-1 ,3 glucans (callose) and mixed beta-1,3- and beta-1,4-glucans and whic h use UDP-D-glucose as substrate. It also differed from xyloglucan syn thase because in the presence of several concentrations of UDP-D-xylos e in addition to GDP D-glucose no xyloglucan was formed. Using either the membrane-bound or the soluble mannan synthase, GDP-D-glucose acted competitively in the presence of GDP-D-mannose to inhibit the incorpo ration of mannose into the polymer. This was not due to an inhibition of the transferase activity but was a result of the incorporation of g lucose residues from GDP-D-glucose into a glucomannan. The kinetics an d the composition of the synthesized glucomannan depended on the ratio of the concentrations of GDP-D-glucose and GDP-D-mannose that were av ailable. Our data indicated that a single enzyme has an active centre that can use both GDP-D-mannose and GDP-D glucose to bring about the s ynthesis of the heteropolysaccharide.