Binding of calcium by organic anions, determined by perturbation of the equilibrium solubility of [14C]calcium oxalate.
Confirmation is needed of the reported binding of calcium ions (Ca2+) by bile salts, which is believed to decrease the activity of free calcium ions [Ca2+] available for precipitation of insoluble calcium salts of organic anions in pigment gallstones. We report a new method to determine the association constants (K'f) of calcium for organic anions, from the perturbation by the added anion of the equilibrium solubilization of calcium [14C]oxalate monohydrate crystals (CaOx*). CaOx* crystals were prepared by stepwise conversion of [14C]oxalic acid to its K+ and Ca2+ salts. Structure and purity were confirmed by X-ray diffraction of the crystals. CaOx* was incubated (37 degrees C, under N2) in 0.15 M NaCl in CO2-free deionized H2O at pH 6.3. Dissolution of CaOx*, estimated by radioassay of the 0.22-micron Millipore filtrate, attained equilibrium at 3 days, with K'sp = [Ca2+] * [Ox=] = 2.34 X 10(-8) M2, calculated using known affinity constants for the soluble complexes of NaOx- (K'NaOx = 3.215 M-1) and CaOx (K'CaOx = 195.0 M-1). Keeping total [Na+] = 0.15 M, we added anions that formed soluble complexes with Ca2+. This decreased free [Ca2+], causing more CaOx* to dissolve in amounts related to the concentration of added anion and its K'f for Ca2+. With this method, K'f values for citrate, malonate and malate were similar to the values we determined with the Ca2+ ion electrode, and to published values obtained with the Ca2+ ion electrode and other methods. The sensitivity of the CaOx method permits determination of K'f values with small quantities and low concentrations of the anions and calcium.
Department of Medicine, Northwestern University, Chicago, IL.
D A
DA
Geller
J D
JD
Ostrow
E W
EW
Moore
L
L
Celic
G H
GH
Nancollas
D002118:Calcium / Q000378:metabolism*; D002129:Calcium Oxalate / Q000378:metabolism*; D002250:Carbon Radioisotopes; D002951:Citrates / Q000378:metabolism*; D019343:Citric Acid; D004566:Electrodes; D007700:Kinetics; D008293:Malates / Q000378:metabolism*; D008314:Malonates / Q000378:metabolism*; D008722:Methods; D012995:Solubility
D002250:Carbon Radioisotopes; D002951:Citrates; D008293:Malates; D008314:Malonates; D002129:Calcium Oxalate; D019343:Citric Acid; C030298:malic acid; C030290:malonic acid; D002118:Calcium