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Articles

-Glucan Synthetases of Plasma Membrane and Golgi Apparatus from Onion Stem ¹

Jaunita I. Kindinger^b and Lawrence Ordin^b

^a Department of Botany and Plant Pathology and Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, Department of Biochemistry, University of California, Riverside, California 92502

Biosynthesis of glucans occurred in cell-free fractions isolated from onion stem (Allium cepa L.) enriched in either dictyosomes or plasma membranes. -1,3- and -1, 4-Glucans were synthesized in differing proportions and at different rates as the concentration of uridine diphosphoglucose or the proportion of dictyosomes or plasma membrane varied. At low (1.5 µM) UDP-glucose concentrations synthesis of alkali-insoluble glucan was correlated with abundance of dicytosomes; most of the substrate utilized by plasma membrane was for glycolipid synthesis. At high (1 mM) UDP-glucose concentration, the synthesis of alkali-insoluble glucans correlated with the abundance of plasma membrane. Substrate enhancement of -1, 4-glucan synthesis in dictyosome fractions was less than proportional to increases in substrate concentration. In contrast, -1, 4-glucan synthesis by plasma membrane was more than proportionately increased. At high substrate concentrations the synthesis of -1, 3-glucans predominated in both dictyosome and plasma membrane fractions. The results show that the capacity to synthesize glucans resides in both Golgi apparatus and plasma membranes of onion stem, but that the plasma membrane has the greatest capacity for synthesis of alkali-insoluble glucans at high UDP-glucose concentrations.

¹ This work was supported by National Science Foundation Grant GB 23183; the Purdue University Joint Highway Research Project to D.J.M.; Environmental Protection Agency, Office of Air Programs Grant R 800870 (formerly AP 00213) to L.O.; a National Science Foundation Postdoctoral Fellowship to C.A.L.; and a National Science Foundation Traineeship to W.J.V. Journal Paper No. 5305 of the Purdue University Agricultural Experiment Station.

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