Structural characterization of three genetic variants of human serum albumin modified in subdomains IIB and IIIA.
Three new genetic variants of human serum albumin have been detected in Italy by routine clinical electrophoresis. Albumin Milano Slow is common in Northern Italy, while albumins Liprizzi and Trieste, which are fast migrating, are rare and local variants. Isoelectric focusing analysis of the CNBr fragments obtained from the carboxymethylated alloalbumins in all cases localized the mutation to fragment CB5 (residues 330-446). The modified CNBr fragments were isolated on a preparative scale and subjected to tryptic digestion. Sequence determination of the abnormal tryptic peptides revealed that all the variants are caused by single point mutations: Trieste, Lys359-->Asn, Milano Slow, Asp375-->His, and Liprizzi, Arg410-->Cys. These results were confirmed by sequence determination of a variant V8 peptide in the case of Trieste, and by DNA sequence analysis for the other two variants. The DNA analysis showed a G-->C transversion at nucleotide position 11969 for albumin Milano Slow, and a C-->T transition at position 13251 for Liprizzi. The latter represents a mutation at a hypermutable CpG dinucleotide site. Albumins Trieste and Milano Slow, as most of the variants thus far described, have mutations involving residues on the surface of the molecule. In contrast, albumin Liprizzi represents the first example of a mutation in the most active binding pocket of the molecule, placed in subdomain IIIA.