Bathocuproine-assisted reduction of copper(II) by human albumin

Human albumin (studied here as the recombinant protein rHA), a copper-bindi ng protein in blood plasma. is shown to reduce Cu(II) to Cu(I) in the prese nce of a Cu(I) chelator, bathocuproinedisulfonate (BD). This reaction was a ccelerated at low pH, when there was little binding of Cu(II) to rHA, The a ddition of a competitive metal ion, Ni(II), or an increase in the concentra tion of ED, enhanced the reduction of Cu(II) to Cu(I). It was concluded tha t the oxidant was the Cu(II) complex of ED. which is likely to bind strongl y to albumin. The free thiol at Cys34 was ruled out as the sole reducing ag ent, since Cys34-blocked albumin also gave rise to Cu(I) in the presence of ED. Reactions with amino acids and peptides suggested that Tyr and possibl y His side-chains are potential reductants, ED and its homologues are frequ ently used as Cu(I)-specific chelators in biological experiments, but the s trong oxidant activity of [Cu(II)(BD)(2)](2-) and its ability to bind to bi ological macromolecules should not be overlooked, and may artificially trig ger/accelerate Cu(II) reduction.