title: The concentrations of free Mg2+ and free Zn2+ in equine blood plasma.
abstract: The enzyme phosphoglucomutase can be used as a metal ion indicator to measure the concentrations of free Mg2+ and free Zn2+ in physiological fluids. In horse plasma, the concentration of free Mg2+ is close to 0.5 mM, whereas that of free Zn2+ is about 2 X 10(-10) M, although numerous physiological roles for Zn2+ have been postulated that would require free Zn2+ concentration orders of magnitude higher than this. A titration of plasma with Zn2+ shows that the fractional increase in free Zn2+ is essentially the same as the fractional increase in total exchangeable Zn2+, and the results are consistent with a model in which essentially all of the Zn2+ in plasma is bound to albumin. Regardless of the model, the buffering capacity of plasma for free Zn2+ is intrinsically low; however, its capacity relative to the total (exchangeable) Zn2+ present is maximal. The implications of this type of buffering for homeostasis of plasma Zn2+ are considered. Treatment of plasma with a strong reducing agent such as dithiothreitol (0.1 mM) substantially increases the apparent binding of Zn2+ and thus reduces the free Zn2+ concentration. However, the concentration of free Zn2+ appears to be insensitive to decreases in the physiological concentrations of reduced glutathione and cysteine. The concentrations of free Zn2+ and free Mg2+ in plasma are similar to those that have been reported for muscle tissue (rabbit). Their ratio is about 4 X 10(-7). The physiological implications of these concentrations are considered. In some cases, if the Zn2+ and Mg2+ complexes of an uncharacterized vertebrate protein exhibit significantly different properties, their relative importance under physiological conditions can be approximated by evaluating those of the mixed complexes present in a solution that contains the physiological concentration of free Mg2+, plus Zn2+ buffered with histidine, at the appropriate pH and ionic strength. Other metal ion/chelon systems that come close to reproducing the concentrations of free Mg2+ and free Zn2+ in horse plasma also are considered.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002413:Cations, Divalent; D004229:Dithiothreitol / Q000494:pharmacology; D006736:Horses / Q000097:blood*; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000097:blood*; D009132:Muscles / Q000201:enzymology; D010733:Phosphoglucomutase / Q000378:metabolism; D010949:Plasma / Q000032:analysis; D011485:Protein Binding; D012709:Serum Albumin / Q000378:metabolism; D013054:Spectrophotometry, Atomic; D015032:Zinc / Q000097:blood*
chemical_list: D002413:Cations, Divalent; D012709:Serum Albumin; D010733:Phosphoglucomutase; D008274:Magnesium; D015032:Zinc; D004229:Dithiothreitol