Subcellular Localization of Spermidine Synthase in the Protoplasts of Chinese Cabbage Leaves

Previous studies on the presence of spermidine synthase (EC 2.5.1.16) in the protoplasts of Chinese cabbage ( Brassica pekinensis var Pak Choy) leaves had detected a small but significant fraction of the enzyme in a crude chloroplast fraction (Cohen, Balint, Sindhu 1981 Plant Physiol 68: 1150-1155). To establish whether this enzyme is truly a chloroplast component, we have isolated purified intact chloroplasts from protoplasts by density gradient centrifugation in silica sols (Ludox AM). Such chloroplasts contained all of the diaminopimelate decarboxylase (EC 4.1.1.20) of the protoplasts, but were essentially devoid of spermidine synthase. Control experiments showed that the latter had not been inactivated under conditions of isolation, purification, and assay of the intact chloroplasts. Isolation and assay of protoplast vacuoles in a further examination of the supernatant fluid containing the enzyme revealed a significant fraction of the enzyme in the vacuole fraction. However this fraction was found to contain similar proportions of a soluble enzyme, glucose 6-phosphate dehydrogenase. It has been concluded that vacuolar fractions are difficultly separable from soluble cytoplasmic material, which is probably the only compartment containing spermidine synthase.