MODULATION OF MAIZE ROOTS H-ATPASE BY SULFATED POLYSACCHARIDES()

Vesicles derived from maize roots retain a membrane bound H+-ATPase th at is able to pump H+ at the expense of ATP hydrolysis. In this work i t is shown that heparin, fucose-branched chondroitin sulfate and dextr an sulfate 8000 promote a shift of the H+-ATPase optimum pH from 6.0 t o 7.0. This shift is a result of a dual effect of the sulfated polysac charides, inhibition at pH 6.0 and activation at pH 7.0. At pH 6.0 dex tran 8000 promotes an increase of the apparent K-m, for ATP from 0.28 to 0.95 mM and a decrease of the V-max from 14.5 to 7.1 mu mol P-i/mg . 30 min(-1). At pH 7.0 dextran 8000 promotes an increase in V-max fro m 6.7 to 11.7 mu mol Pi/mg . 30 min(-1). In the presence of lysophosph atidylcholine the inhibitory effect of the sulfated polysaccharides ob served at pH 6.0 was not altered but the activation of pH 7.0 decrease d. It was found that in the presence of sulfated polysaccharides the A TPase became highly sensitive to K+ and Na+. Both the inhibition at pH 6.0 and the activation promoted by the polysaccharide were antagonize d by monovalent cations(K+ > Na+ much greater than Li+).