Anion stimulation of enzymes - physiological or niet?

[Dan Diaz]

i work with an e. coli exonuclease which is stimulated about ten-fold by 50
mM phosphate.  this is hardly physiological.  is the enzyme simply not as
active in vivo as in test tubo?  can other anions substitute in vivo?  any
suggestions for physiological anions to try in tubo?