Modulation of maize roots H(+)-ATPase by sulfated polysaccharides.

Vesicles derived from maize roots retain a membrane bound H(+)-ATPase that is able to pump H+ at the expense of ATP hydrolysis. In this work it is shown that heparin, fucose-branched chondroitin sulfate and dextran sulfate 8000 promote a shift of the H(+)-ATPase optimum pH from 6.0 to 7.0. This shift is a result of a dual effect of the sulfated polysaccharides, inhibition at pH 6.0 and activation at pH 7.0. At pH 6.0 dextran 8000 promotes an increase of the apparent K(m) for ATP from 0.28 to 0.95 mM and a decrease of the Vmax from 14.5 to 7.1 mumol Pi/mg x 30 min-1. At pH 7.0 dextran 8000 promotes an increase in Vmax from 6.7 to 11.7 mumol Pi/mg x 30 min-1. In the presence of lysophosphatidylcholine the inhibitory effect of the sulfated polysaccharides observed at pH 6.0 was not altered but the activation of pH 7.0 decreased. It was found that in the presence of sulfated polysaccharides the ATPase became highly sensitive to K+ and Na+. Both the inhibition at pH 6.0 and the activation promoted by the polysaccharide were antagonized by monovalent cations (K+ > Na+ > > Li+).