Equine Hepatic and Renal Metallothioneins

An improved, simplified, and greatly shortened isolation procedure for metallothionein from tissue extracts using gel filtration (Sephadex G-SO) and DEAE-cellulose chromatography is presented. When isolated by this procedure, the molecular weight of both the liver and kidney metalloproteins is found to be about 6600, as determined by equilibrium centrifugation and gel filtration. The amino acid composition of both proteins is also closely similar. Their distinctive features include the extremely high content of cysteine which comprises about 33% of all amino acid residues, and the absence of phenylalanine, tyrosine, tryptophan, and histidine. In addition, both proteins show substantial microheterogeneity even when isolated from the organ(s) of a single animal. Based on a molecular weight of 6000, the nearest integral composition for a total of 61 amino acids is Lyss Argl Aspr Thr, Ser, Glu, Proa Gly, Alaa CyszO Valz Met1 for both renal and hepatic proteins. All 20 cysteinyl residues are titratable with silver ions and hence the protein contains no disulfide bonds. The total metal content consisting of cadmium, zinc, and copper is close to 6 g atoms per mole for both liver and kidney metallothioneins; the liver protein contains predominantly zinc whereas that from kidney contains more cadmium. Thus, for each metal atom bound on the average 3 cysteinyl residues are available for binding.