Exchange of Major Cations and Anions in Cells

The major small cations and anions in cells which undergo ready exchange are shown in Table 1. They may associate with one another in rapid exchange or bind to surfaces of proteins, other polymers, precipitates or membranes and exchange rapidly or slowly.

Consider a magnesium protein such as Mg2 + parv-albumin, where parvalbumin is a common anionic metal-ion-binding protein in muscle cells. When the protein, which is magnesium bound in the cell, is exposed to calcium, the reaction of exchange occurs: Ca2 + parvalbumin is formed and Mg2 + is freed. If the initial protein complex had been Na + parvalbumin, then replacement by Ca2 + would have been electrogenic, unless of course two Na + ions had been bound originally.

A second example of exchange on the surface of a polymer is provided by polynucleotides such as RNA and DNA. These polymers have anionic phosphate backbones and their negative charges are balanced, not entirely randomly, by exchanging cations, including particularly K+, Mg2 + and ammonium derivatives. The positive charge, like the negative charge, in an exchanging system can also be carried on a large molecule and in biology there are many polyamines and positively charged proteins, such as histones, which bind DNA. They can bind and exchange anions such as HPO4" and SO2".

A further example of biological metal ion exchange in one aqueous solution now involving a solid precipitate is the case of bone, which we can write as (Ca2 + )2(OH" )PO4" for simplicity. Bone scavenges many other cations by exchange and contains considerable amounts of Mg2 + and even Al3 + in place of Ca2 +. Bone also exchanges Ca2 + for protons (H+ ). This mineral is additionally an anion-exchanging solid in which some OH" is replaced by F" (a process used in the protection of teeth). To some extent, a pair of OH" + PO3" can be replaced by 2CO23" anions. Notice that exchange can be electrogenic when charge is left on the surface of the bone mineral.

The last example of this relatively simple exchange is at the surface of membranes. All biological membranes carry negative charge due to covalently bound phosphate or carboxylate groups. Since the charges are quite close-packed, the membranes have a considerable affinity for cations and all membranes have associated K+ and Mg2+ (mainly internally) and Na+ and Ca2+ (mainly externally) associated with their surfaces. The outermost coats of cell walls in Gram-negative bacteria are also anionic and are stabilized by exchangeable cation, often Ca2 +, incorporation.

Table 1 Major small cations and anions of cells

Cation Anion

Na + largely rejected

CI3 largely rejected

K+ cytoplasmic

HPO243 cytoplasmic

Mg2+ cytoplasmic

SO243 cytoplasmic

Ca2+ rejected (vesicular)

HCO33 balanced

RNH+ cytoplasmic

RCO23 cytoplasmic ROPO3" cytoplasmic

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