Calcium and Adenosine Triphosphate Binding to Renal Membranes

Calcium binds to membranous structures isolated from rabbit kidney cortex homogenates. The binding is enhanced by ATP and Mg⁺⁺ in combination. Other nucleotides, ITP and GTP, do not have this property. In contrast to similar preparations of nerve and muscle, the binding is not augmented by oxalate (3–100 mM). Also, binding of calcium cannot be correlated with ATP hydrolysis. p-Chloromercuribenzoate and the mercurial diuretic agent mercaptomerin inhibit the binding of calcium. This system can be distinguished from the binding of calcium by mitochondria by lack of azide inhibition and by failure of ADP-succinate to substitute for ATP. ¹⁴C- and γ-³²P-labeled ATP bind to the renal membranes in the absence of calcium, but only the ³²P binding increases when calcium is added. The ratio of ³²P bound to ⁴⁵Ca bound is 2:1. The above data are consistent with a hypothesis that calcium is metabolically bound to renal membranes and that this binding is associated with membrane phosphorylation. Such a formulation may have pertinence to the conformational state of renal membranes and subsequent permeability characteristics. It also allows for the concept that membrane stability requires metabolic participation, as well as calcium ions.