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Tissue Slice and Particulate -Glucan Synthetase Activities from Pisum Epicotyls ¹

Department of Biology, McGill University, 1205 McGregor Avenue, Montreal, Quebec, Canada H3A 1B1

-Glucan synthetase activity in growing regions of pea (Pisum sativum L.) epicotyls was assayed by supplying UDP-glucose to particulate fractions of tissue homogenates or to thin tissue slices. Particulate fractions are less active in forming alkali-insoluble glucan than slices from the same tissue, although many kinetic characteristics (pH and Mg²⁺ optimum, apparent K_m) are similar for the two systems. Synthesis by tissue slices progresses linearly without lag period for at least an hour and is proportional to cut surface area. It is much more rapid from UDP-glucose than from glucose, glucose-1-P, or sucrose. Tests with plasmolyzing agents and trypsin support the conclusion that synthesis from UDP-glucose by slices occurs at accessible surfaces of cut cells. Analyses of glucan products by GLC of partially methylated and acetylated derivatives and by hydrolysis with various -glucanases all show that both -1,3 and -1,4 linkages are formed by particulate fractions and slices at substrate concentrations ranging from micro- to millimolar. -1,4 Linkages predominate at low substrate (5 µM) concentration. Kinetic data indicate that the capacity to synthesize -1,3-glucan is substrate-activated, and this product predominates in preparations supplied with high (5 mM) substrate.

¹ This study was supported by grants to G. M. and scholarship to Y. R. from the National Research Council of Canada and the Quebec Ministry of Education.

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