Albumin sulfhydryl groups

Serum albumin has 35 cystein residues which are found as 17 intrachain disulfide linkages and one free sulfhydryl group. Except for the immunoglobulins, serum albumin is the largest milk protein (Walstra and Jenness 1984). 

Figure 5 Covalent coupling of cyclic peptide moieties to human serum albumin (HSA). The depicted cyclic peptide, C SRNLIDC, in which C denotes the cyclizing cysteine residues, mimics the receptor binding site of PDGF-BB. First, a sulfhydryl group is introduced to the cyclic peptide by a reaction with succinimide-acetyl thioacetate (SATA). The primary amino groups of lysine in HSA are derivitized with maleimide-hexoyl-At-hydroxysuccinimide ester (MHS). Subsequently, the cyclic peptide is coupled to HSA. In this latter reaction, hydroxyl amine is used to remove the protecting acetate group from the sulfhydryl group of the cyclic peptide.

Like cadmium, mercury(II) has a strong affinity for sulfhydryl groups in proteins, enzymes, hemoglobin, and serum albumin. Because of the abundance of sulfhydryl groups in active sites of many enzymes, it is difficult to establish exactly which enzymes are affected by mercury in biological systems. 

However, if a reagent would react only with a single site on a macromolecule, then a chemically homogeneous product having one chelator per protein molecule could be formed. Alkylation of the single sulfhydryl group of albumin with a haloacetyl reagent probably comes reasonably close to this situation. 

The crystal structure of human albumin located Cysteine-34 at the turn between helices h2 and h3 with the side chain sulfhydryl group oriented toward the protein interior, consistent with EPR studies suggesting that it is 950 pm below the surface. Sadler has demonstrated by H NMR studies that the cys-34 residue must move outward from the crevice created by the helices before it can react to form disulfide bonds or bind to Et3PAu+ derived from auranofin. These structural observations are consistent with the kinetic mechanism for the reactions of albumin with auranofin and its triisopropylphosphine analogue, which revealed a slow crevice opening reaction in equilibrium between open and closed forms of albumin. The kinetic model accounts for a process that is first order in protein when the auranofin is present in excess. 

Absorbed Cd bound to red blood cells and serum albumin (binding to sulfhydryl groups on proteins is especially strong). 

DeMastcr, E, G Quast, B. j., Redferm, B., and Nagasawa, H. T (1995). Reaction of nitric oxide with the free sulfhydryl group of human senjm albumin yields a stilfenie acid and nitrous oxide. Bioc u mistry 34,11494-11499. 

The purest available specimens of albumin are now known to be homogeneous (F15), except in one unique respect. The native protein appears to contain only 0.6-0.8 mole SH per mole albumin, i.e., in approximately 3 of 10 molecules there is no detectable reduced sulfhydryl group. Two explanations for this phenomenon have been put forward. In the first, the absent SH is thought to be the result of a sulfhydryl-disulfide exchange in the presence of cystine ... 

H6. Hartley, R. W., Peterson, E. A., and Sober, H. A., The relation of free sulfhydryl groups to chromatographic heterogeneity and polymerization of bovine plasma albumin. Biochemistry 1, 60-68 (1962). 

Mercurated fluorescein derivatives, which were covalently linked to the sulfhydryl group of bovine serum albumin (BSA), showed induced Cotton effects in the region of the dye absorption, which disappeared in 8 M urea (304). The dependence on />H of the Cotton effects indicated... 

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