Caloxin: a novel plasma membrane Ca2+ pump inhibitor

Plasma membrane (PM) Ca2+ pump is a Ca2+-Mg2+-ATPase that expels Ca2+ from cells to help them maintain low concentrations of cytosolic Ca2+. There are no known extracellularly acting PM Ca2+ pump inhibitors, as digoxin and ou abain are for Na+ pump. In analogy with digoxin, we define caloxins as extr acellular PM Ca2+ pump inhibitors and describe caloxin 2A1. Caloxin 2A1 is a peptide obtained by screening a random peptide phage display library for binding to the second extracellular domain (residues 401-413) sequence of P M Ca2+ pump isoform 1b. Caloxin 2A1 inhibits Ca2+-Mg2+ ATPase in human eryt hrocyte leaky ghosts, but it does not affect basal Mg2+-ATPase or Na+-K+-AT Pase in the ghosts or Ca2+-Mg2+-ATPase in the skeletal muscle sarcoplasmic reticulum. Caloxin 2A1 also inhibits Ca2+-dependent formation of the 140-kD a acid-stable acylphosphate, which is a partial reaction of this enzyme. Co nsistent with inhibition of the PM Ca2+ pump in vascular endothelium, calox in 2A1 produces an endothelium-dependent relaxation that is reversed by N-G -nitro-L-arginine methyl ester. Thus caloxin 2A1 is a novel PM Ca2+ pump in hibitor selected for binding to an extracellular domain.