Inhibition of plasma membrane Ca2+-ATPase by heparin is modulated by potassium.
Heparin is related to several protein receptors that control Ca2+ homeostasis. Here, we studied the effects of heparin on the plasma membrane Ca2+-ATPase from erythrocytes. Both ATP hydrolysis and Ca2+ uptake were inhibited by heparin without modification of the steady-state level of phosphoenzyme formed by ATP. Calmodulin did neither modify the inhibition nor the binding of heparin. Inhibition by heparin was counteracted by K+ but not by Li+. This effect was extended to other sulfated polysaccharides with high number of sulfate residues. Hydrolysis of p-nitrophenylphosphate was equally inhibited by heparin. No evidence for enzyme uncoupling was observed: Ca2+ uptake and ATP hydrolysis remained tightly associated at any level of heparin, and heparin did not increase the passive Ca2+ efflux of inside-out vesicles. Vanadate blocked this efflux, indicating that the main point of Ca2+ escape from these vesicles was linked to the Ca2+ pump. It is discussed that sulfated polysaccharides may physiologically increase the steady-state level of Ca2+ in the cytosol by inhibiting the Ca2+ pumps in a K+ (and tissue) regulated way. It is suggested that heparin regulates the plasma membrane Ca2+-ATPase by binding to the E2 conformer.
Felix
Carla F
CF
Instituto de Bioquímica Médica, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, Ilha do Fundão, Rio de Janeiro, RJ 21941-590, Brazil.
Oliveira
Vanessa H
VH
Moreira
Otacilio C
OC
Mignaco
Julio A
JA
Barrabin
Hector
H
Scofano
Helena M
HM
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors* / Q000097:blood; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004910:Erythrocyte Membrane / Q000187:drug effects* / Q000201:enzymology*; D006493:Heparin / Q000494:pharmacology*; D006706:Homeostasis / Q000187:drug effects; D066298:In Vitro Techniques; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D011134:Polysaccharides / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D013552:Swine
D004791:Enzyme Inhibitors; D011134:Polysaccharides; D000255:Adenosine Triphosphate; D006493:Heparin; D008094:Lithium; D000252:Calcium-Transporting ATPases; D011188:Potassium