Immobilised cooper(II) ion-affinity chromatography of natural mutants of human serum albumin and proalbumin

Interaction of natural mutants of human serum albumin and proalbumin with chelated and immobilised Cu(II) ions (IDA-Cu(II)) has been studied by chromatography. All the studied variants bound to IDA-Cu(II) columns under the experimental conditions employed (pH 6.8; 0.5M NaCl) and were displaced by affinity elution with imidazole. Variants with a propeptide, i.e. the proalbumins (proAlb) Varese, Blenheim, and Kaikoura, and those with an extra Arg residue (Arg-1) at the N-terminus, i.e. the albumins (Alb) Redhill and Arg-Alb, had the highest affinities for immobilised Cu2+. Binding to IDA-Cu(II) was moderately increased by modification of the C-terminal region of the albumin molecule (i.e. Albs Venezia and Catania) but was not affected by structural changes at the N-terminal end (i.e. Alb Blenheim) and in subdomains IIB (i.e. Albs Herborn and French Reading) and IIIB of the molecule (i.e. Alb Casebrook). The different metal-binding of these variants was further confirmed in other studies using IDA-Cu(II) chromatography with ammonium chloride as eluent. The variant Arg-Alb was displaced from IDA-Cu(II) only on protracted irrigation of the columns with ammonium chloride (1 M) whereas the three proalbumins studied could not be displaced. All the other albumin variants were readily eluted from the columns with ammonium chloride (1 M), although their retention behaviur varied. Analysis of the (pro)albumin variants for Cu2+ content, after their passage through IDA-Cu(II) columns, showed that these proteins differed in their ability to scavenge Cu2+ from its immobilised chelate, IDA-Cu(II). The different scavenging capabilities seem to be related to different affinities of these variants with regard to primary binding of free Cu2+ ions. A molecular explanation of the different IDA-Cu(II) binding of the tested (pro)albumins is proposed; this makes use of the possibility given by available views of the crystal structure of human serum albumin. More practically, the possibility of resolving some of the tested (pro)albumin variants from their normal counterpart (endogenous albumin) by IDA-Cu(II) chromatography could aid the separation and further purification of these variants.