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

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Abstract
.beta.-Glucan synthetase activity in growing regions of pea (P. 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 Km) are similar for the 2 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 .beta.-glucanases all show that both .beta.-1,3 and .beta.-1,4 linkages are formed by particulate fractions and slices at substrate concentrations ranging from micro to millimolar. .beta.-1,4 Linkages predominate at low substrate (5 .mu.M) concentration. Kinetic data indicate that the capacity to synthesize .beta.-1,3-glucan is substrate-activated, and this product predominates in preparations supplied with high (5 mM) substrate.