Effects of Germination on NA+-K+-stimulated Adenosine 5′-Triphosphatase and ATP-dependent Ion Transport of Isolated Membranes from Cotyledons

A purified membrane fraction featuring ATPase activity was isolated from cotyledon tissue of Phaseolus vulgaris at different stages of germination. The fraction is enriched in both basal and Na(+)-K(+)-stimulated ATPase and is relatively free of contamination by fragments of mitochondrial membrane and microsomes. The isolated membranes have been tentatively identified as partially purified plasma membrane.The specific activities of the basal- and cation-sensitive ATPases were high in membrane preparations from young cotyledon tissue but decreased with advancing senescence. Electron microscopy of the preparation showed that the isolated membranes were of primarily vesicular conformation. These vesicles proved to be capable of extruding Na(+) and K(+) in the presence of ATP. Moreover, the degree of ATP-dependent extrusion varied during germination in a pattern that resembled variations during the same period in the Na(+)-K(+)-stimulated ATPase of the isolated fraction. This indicates that as the level of cation-sensitive ATPase on the membrane rises or falls, there is a corresponding change in the ability of the membrane to actively translocate Na(+) and K(+).