[Sucrose metabolizing enzymes in roots].
In tomato, pea, and maize roots, the presence of sucrose synthetase (E.C. 2.4.1.13), sucrose phosphate synthetase (E.C. 2.4.1.14), and both the alkaline and acid sucrases (E.C. 3.2.1.? and E.C. 3.2.1.26) could be demonstrated. The activity of these enzymes was measured directly on root slices suspended in the incubation mixture, which also contained 10% dimethyl sulfoxide or 5% ethyl acetate. These two additives increase the permeability of the membranes, thus allowing the entry of substrates to the site where the enzymes are located and favouring the release of products into the medium. The values found by measuring the activities of sucrose synthetase and the alkaline sucrase on tomato root were quite comparable to those obtained using enzymic preparations of the same tissue. In the case of the weakly active sucrose phosphate synthetase and the very active acid sucrase, the comparison of the two methods showed that losses of activity occurred during the preparation of the enzymes. By separating stele from cortex in tomato and maize roots, it could be shown that sucrose phosphate synthetase is located only in the stele, while the other enzymes are unevenly distributed between the two tissues, the sucrases predominating in the cortex. Sucrose phosphate synthetase and sucrose synthetase are most active in the region with root hairs of maize roots, while the activity of both sucrases is higher in the region near the apex. The highest invertase activity was measured in the apex of stem-borne adventitious roots that had not yet penetrated the soil. In older roots, sucrose phosphate synthetase disappears and the activity of all the other enzymes diminishes. It is suggested that the high activity of sucrose synthetase is present in the regions where an active synthesis of cell wall substances from glycosyl nucleotides is taking place, while the sucrases provide the initial substrates for most of the other pathways.
Fachbereich Biologie der Technischen Hochschule, Schnittspahnstr. 3-5, D-6100, Darmstadt, Federal Republic of Germany.
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Vieweg