Estimating calcium vs magnesium binding affinity - (Dec/05/2015 )

Trisodium citrate is commonly used as a medical anticoagulant and works by binding divalent cations.  When the concentration of free calcium falls below about 225 nmol, coagulation cannot proceed.

Citrate also binds magnesium and I have found plenty of references that state when performing regional anticoagulation with citrate, such as with 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.  The reason I need to know is that I want to chelate enough Ca to reduce ionized calcium from the typical physiologic range of 1,100-1,400 nmols to down to a specific level of 350-400 nmols, but no further.  I will have a known volume of bone marrow and the staring concentrations for Ca and Mg, so all I need to know is that amount of citrate to add to yield the desired concentration.

The problem is that I do not know how aggressively citrate will bind to Ca vs. Mg.  The part that binds to Mg will be unavailable to bind to Ca, so I will need to add more than the theoretical amount required to reduce just the Ca concentration, but if Mg is a weak competitive binder I run the risk of reducing the Ca too far.  I could figure this out via experimentation, but acquiring fresh bone marrow samples to play with is a non-trivial procedure.

Does anybody have a suggestion on how to estimate the amount of Ca and Mg that will be bound for a given amount of citrate?  Even better, if anybody can point me to a reference article that discusses this in detail that would be great.  Several hours of searching on Google and PubMed have not been fruitful.

I think you need to determine this experimentally, especially since you want to do this in a very complex environment. You'll need fast, easy and accurate ways of assessing Ca++ and Mg++ concentrations in complex media. I don't know what those are, but I assume they exist. A major challenge will be determining where you want to do the measurement, since the cells, if broken open, will change things dramatically.

these publications (linked) may help you:

http://pubs.acs.org/doi/abs/10.1021/j100819a045

and

http://www.ncbi.nlm.nih.gov/pubmed/1393620

there are programs available (see the second publication) to determine the concentrations using the constants in the literature. i had one years ago but citrate was not one of the ligands available (and i didn't require it or i would have added it to the program).

Thanks for the replies.  I appreciate it.