Calcium vs magnesium binding

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      Trisodium citrate is commonly used as an anticoagulant and works by binding divalent cations. When free calcium falls below about 225 nmols, coagulation cannot proceed.

      Citrate also binds magnesium, and I have found plenty of reference that state that when performing regional anticoagulation with citrate (like in a dialysis circuit) the patient will lose both Ca++ and Mg++. What I have been unable to determine from the literature is what the relative binding affinities are for each ion. The reason I need to know is that I want to chelate free calcium from the typical physiologic level of 1,100-1,400 nmols down to a specific level, roughly 350-400 nmols, and no further. I have a given volume of bone marrow aspirate and I know the starting concentrations of Ca++ and Mg++, all I need to know is how much citrate to add to chelate the desired amount of Ca to yield the desired concentration.

      The problem is that I don’t know how aggressively citrate binds to Ca vs Mg. Obviously the part that binds to Mg will be unavailable to bind Ca, so I need to add somewhat more than the theoretical amount for Ca alone, but if Mg is a weak competitive binder I don’t want to add too much citrate and drop my Ca concentration too far. I could figure this out by experimentation, but obtaining fresh bone marrow samples to play with is not a trivial procedure nor inexpensive.

      Does anybody have a suggestion on how to figure out how much of each electrolyte will be bound for a given amount of citrate or, even better, point me to a reference article. So far Google and PubMed, or at least my searching skills, have let me down.

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