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.
Geller
D A
DA
Department of Medicine, Northwestern University, Chicago, IL.
Ostrow
J D
JD
Moore
E W
EW
Celic
L
L
Nancollas
G H
GH
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