Lanthanum as a calcium-substituting ion for binding to sarcoplasmic reticulum ATPase.
Ca2+ binding and internalization in sarcoplasmic reticulum ATPase can be investigated by the use of La3+ as a Ca2+ analog. Displacement kinetics of Ca2+ bound by La3+ in native vesicles is a slow biphasic process (k1 = 0.55 s-1 and k2 = 0.05 s-1) that is consistent with the existence of two Ca2+ binding populations whereas in leaky vesicles there appears to be a single population (k = 0.57 s-1). Rapid quench experiments demonstrate that Ca2+ internalization occurs with an initial burst (approximately 8 nmol/mg protein) associated with the presence of a phosphate-donor substrate in the reaction medium. While acid quenching for measurements of phosphoenzyme is instantaneous, La3+ quenching allows completion of one catalytic and transport cycle due to the slow La3+ exchange with Ca2+. This explains the apparent inconsistencies in the kinetics and stoichiometry of phosphoenzyme formation and Ca2+ internalization that are observed under certain experimental conditions.
Fernandez-Belda
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Departamento de Bioquimica y Biologia Molecular, Universidad de Murcia, Spain.
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D007811:Lanthanum / Q000494:pharmacology*; D010766:Phosphorylation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013997:Time Factors
D007811:Lanthanum; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D002118:Calcium