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Subcellular Localization of 2-(-D-Glucosyloxy)-Cinnamic Acids and the Related -glucosidase in Leaves of Melilotus alba Desr. ¹

Department of Biochemistry and Biophysics, University of California, Davis, California 95616, Centre de Physiologie Végétale, Laboratoire Associé au Centre National de la Récherche Scientifique No. 241, Université Paul Sabatier, 31062, Toulouse Cédex, France

The distribution of the glucosides of trans- and cis-2-hydroxy cinnamic acid and of the -glucosidase which hydrolyzes the latter glucoside was examined in preparations of epidermal and mesophyll tissue obtained from leaves of sweet clover (Melilotus alba Desr.). The concentrations of glucosides in the two tissues were about equal when compared on the basis of fresh or dry weight. Inasmuch as the epidermal layers account for no more than 10% of the leaf volume, the mesophyll tissue contains 90% or more of the glucosides. Vacuoles isolated from mesophyll protoplasts contained all of the glucosides present initially in the protoplasts.

The specific activities of the -glucosidase in the two tissues were also similar; thus, most of the enzyme is contained in mesophyll tissue. However, the amount of enzyme in mesophyll protoplast extracts amounts to only 1 to 2% of the activity present in leaf homogenates when chlorophyll was the basis for comparison. (This small amount of coumarin--glucosidase present in protoplasts is not associated with chlorophyll-containing fractions.) In contrast, 90% of the uridine diphosphate glucose-o-coumaric acid glucosyl transferase activity present in leaf homogenate was recoverable in protoplasts prepared from intact leaves. Such results indicate that most of the coumarin--glucosidase in M. alba leaves is located in the extracytoplasmic space. Only a small fraction (7%) of this extra cytoplasmic -glucosidase was associated with individual cells or cell clusters isolated from clover leaves.