title: Maize tonoplast PP(i)-dependent H(+)/Ca(2+) exchange: two K(s) for Ca(2+) and inhibition by thapsigargin.
abstract: Maize root tonoplasts are able to accumulate Ca(2+) using the energy derived from the H(+) gradient formed during PP(i) hydrolysis. Oxalate increases 6- to 10-fold the amount of Ca(2+) accumulated by tonoplast. Two apparently different K(s) values for Ca(2+) with values of 0.36 and 4.70 microM were detected when oxalate was included in the medium and the free Ca(2+) concentration in the medium was buffered with the use of EGTA. Binding of Ca(2+) to the outer surface of tonoplasts inhibits the outflow of Ca(2+) previously accumulated by the tonoplast, half-maximal inhibition being observed in presence of 1 microM Ca(2+). Thapsigargin, a specific inhibitor of Ca(2+)-ATPase, inhibits the Ca(2+) uptake driven by H(+) gradient but does not inhibit the hydrolysis of PP(i) nor the formation of a H(+) gradient.
authors.affiliation: Departamento de Bioqui;mica Médica, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro 21941-590, Brazil.
authors.forename: Wagner S
authors.initials: WS
authors.lastname: da-Silva
authors.forename: Flavio M
authors.initials: FM
authors.lastname: Bomfim
authors.forename: Antonio
authors.initials: A
authors.lastname: Galina
authors.forename: Leopoldo
authors.initials: L
authors.lastname: de Meis
mesh_terms: D017920:Antiporters / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D027682:Cation Transport Proteins; D011756:Diphosphates / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000201:enzymology; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D010070:Oxalates / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology*; D003313:Zea mays / Q000201:enzymology*
chemical_list: D017920:Antiporters; D002135:Calcium-Binding Proteins; D027682:Cation Transport Proteins; D011756:Diphosphates; D004791:Enzyme Inhibitors; D010070:Oxalates; C027614:calcium-hydrogen antiporters; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium