Isolation and two-dimensional 1H-NMR of peptide [1-24] of dog serum albumin and studies of its complexation with copper and nickel by NMR and CD spectroscopy

Structural and biochemical characterisation of Co2+-binding sites on serum albumins and their interplay with fatty acids

Serum albumin-Co²⁺ interactions are of clinical importance. They play a role in mediating the physiological effects associated with cobalt toxicity and are central to the albumin cobalt binding (ACB) assay for diagnosis of myocardial ischemia. To further understand these processes, a deeper understanding of albumin-Co²⁺ interactions is required. Here, we present the first crystallographic structures of human serum albumin (HSA; three structures) and equine serum albumin (ESA; one structure) in complex with Co²⁺. Amongst a total of sixteen sites bearing a cobalt ion across the structures, two locations were prominent, and they relate to metal-binding sites A and B. Site-directed mutagenesis and isothermal titration calorimetry (ITC) were employed to characterise sites on HSA. The results indicate that His9 and His67 contribute to the primary (putatively corresponding to site B) and secondary Co²⁺-binding sites (site A), respectively. The presence of additional multiple weak-affinity Co²⁺ binding sites on HSA was also supported by ITC studies. Furthermore, addition of 5 molar equivalents of the non-esterified fatty acid palmitate (C16:0) reduced the Co²⁺-binding affinity at both sites A and B. The presence of bound myristate (C14:0) in the HSA crystal structures provided insight into the fatty acid-mediated structural changes that diminish the affinity of the protein toward Co²⁺. Together, these data provide further support for the idea that ischemia-modified albumin corresponds to albumin with excessive fatty-acid loading. Collectively, our findings provide a comprehensive understanding of the molecular underpinnings governing Co²⁺ binding to serum albumin.

Further characterization of the N-terminal copper(II)- and nickel(II)-binding motif of proteins. Studies of metal binding to chicken serum albumin and the native sequence peptide

We have investigated the Cu(II)- and Ni(II)-binding properties of chicken serum albumin (CSA) and of the native sequence tripeptide derived from the N-terminus of this protein. Spectrophotometric and equilibrium dialysis experiments demonstrate that Cu(II) and Ni(II) bind non-specifically at the N-terminus of CSA. Proton displacement studies show that the histidine residue in the fourth position of the protein does not appear to participate in the binding of the two metals. Consistent results were obtained with the native sequence tripeptide L-aspartyl-L-alanyl-L-glutamic acid N-methylamide. The results presented here demonstrate that neither the glutamic acid residue in the third position nor the histidine in the fourth position participate in the binding of Cu(II) and Ni(II) to CSA. It is known, however, that a number of other albumins with a histidine residue in the third position possess high-affinity Cu(II)- and Ni(II)-binding sites. Our results provide further evidence that the N-terminal Cu(II)/Ni(II)-binding motif requires a histidine at the third position in order to bind Cu(II) and Ni(II) specifically.

Restriction of the participation of copper in radical-generating systems by zinc

Zinc was able to reduce the availability of copper for several radical-generating reactions: lucigenin-amplified chemiluminescence due to copper and hydrogen peroxide: copper-dependent ascorbate oxidation and its concomitant oxygen consumption: and copper-dependent benzoate hydroxylation. This was the case both in the presence of bovine serum albumin (when most zinc was protein-bound) and in its absence (when zinc was 'available'). Competition between zinc and copper for binding to the fluorophore calcein was also examined, and this allowed assessment of copper availability in several circumstances. Competition between copper and zinc for binding to biological components seems to be a rather general phenomenon, and thus zinc is commonly a protective entity, restricting free radical generation.

Stimulatory and inhibitory actions of proteins and amino acids on copper-catalysed free radical generation in the bulk phase

The effect of a variety of proteins and amino acids was investigated on oxygen free radical activity as assessed by copper/hydrogen peroxide induced benzoate hydroxylation as well as copper-catalysed ascorbate autoxidation. Serum albumins from a variety of species (human, bovine and dog) had both inhibitory and stimulatory effects depending on the molar copper to protein ratio; low ratios were inhibitory and high stimulatory. Some other proteins tested (lysozyme, soybean trypsin inhibitor and conalbumin) also had dual (inhibitory and stimulatory) effects, as did both histidine and polyhistidine, but all effects occurred at different molar ratios presumably dependent on the relative affinities for the copper ions. In contrast, metallothionein and caeruloplasmin, proteins specialised to bind copper in vivo had no stimulatory effects. In this paper we show that in addition to their fairly well documented inhibitory effects, under certain conditions some proteins also stimulate radical reactions. The possible role of this phenomenon in vivo is discussed.