ABSTRACT
Two main factors determine whether metals interact with each other: concentration and affinity. Lead, calcium, and zinc exist in a variety of chemical forms in biological systems. They are
all divalent cations, but they also form complexes with a number of inorganic (e.g., OH-, H2P04-) and organic anions (e.g., histidine, cysteine, citrate) as well as with proteins. Protein-binding can be nonspecific (e.g., to albumin) or specific (e.g., Zn2•-metallothionein). Any one of the chemical complexes of Pbl+ may be responsible for a particular toxic effect, so it is important to know about the speciation of lead, i.e.1 the concentrations of the different species present. These depend upon the concentrations of the various ligands that bind lead and competing metals, and the equilibrium constants for the metal-ligand reactions, provided those reactions are at equilibrium. Most metalligand reactions occur quickly in aqueous media, and can be considered to be at equilibrium. Some exceptions are known, e.g., Zn2• is effectively inexchangeable when it is bound to the enzyme carbonic anhydrase, or to the plasma protein a2-macroglobulin, but Ca2• and Pbl+ seem to be in equilibrium with their ligands, as far as is known.
