Effects, in vitro, of polar lipids on K+ Mg2+- dependent ATPase of membrane preparations in radish seeds

Recent studies have shown that the first phases in the germination of radish seeds are accompanied by the activation of the abiIity to take up K+. Further findings suggest that a Mg - K -dependent AT Pase DCCD-sensitive, found in membrane preparations, may be invoIved in the K uptake mechanism of the seed. This activity increases right from the earliest phases of germination ( 1 ). Finally, data obtained in this laboratory in studies of this material, and findings in the literature on other materials show that during the earliest phases of germination there are important changes in the polar lipids composition of the seed. This paper describes studies carried out to determine whether the DCCD-sensitive Mg --K -dependent ATPase found in membrane preparations requires lipids for its activity in a way similar to that already known for other membrane ATPases. A further aim was to determine whether variations in polar Iipids contents found during early germination phases have differing ability to stimulate the AT Pase. The data show that the DCCD-sensitive Mg -K -dependent ATPase present in membrane preparations (the mitochondrial contamination was below 2%) increases by 290% during the first 24h of germination (imbibed seeds:0.25 umoles Pi hydrolyzed/h /g fresh weight; seeds at 24h of germination: 0.98 umoles Pi hydroIyzed /.h/ g initial fresh weigth ). Membrane reparations extracted from ungerminated seeds and from seeds germinated for 24 h ,lost practically all activity when are partially delipidized. When polar lipids (extracted from radish seeds) were added as Iipomes to such preparations, the ATPase activity was almost totally restored. Lastly, polar lipids extracted from seeds which had germinated for 24 h were found to be rrore effective in activating the AT Pase than Iipids extracted from ungerminated seeds, both when the ATPase was extracted from ungerminated seeds and when it was from seeds germinated for 24 h. The data show that ATPase from seed membranes requires lipids for its activity. Moreover the data showing that ATPase activity is greater when reconstitution is carried out with lipids from seeds germinated for 24 h, suggest that variation of lipids composition of the membrane during germination may be an important factor in determining the rise in ATPase activity during early phases of germination. ( 1 ) Cocucci,S. and Cocucci, M., Plant Science Letters 10,85 (1977). 13.5.