Albumin Hawkes Bay; a low level variant caused by loss of a sulphydryl group at position 177 |
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Authors: | Stephen O Brennan Andrew P Fellowes |
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Institution: | Molecular Pathology Laboratory, Clinical Biochemistry Unit, Pathology Services, Christchurch New Zealand |
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Abstract: | A slow migrating minor albumin component, representing 5% of total circulating albumin, was detected by routine serum protein electrophoresis and immunofixation. After treatment with 5 mM dithiothreitol the abnormal component was found to migrate normally suggesting the attachment of some component to the free thiol at position 34. However, purification and analysis by SDS-PAGE showed that the abnormal component had a slightly lower apparent molecular weight than normal albumin. Limited tryptic cleavage indicated the abnormal site to be in the N-terminal third of the molecule. HPLC analysis of tryptic peptides from this domain showed the presence of a new peptide of sequence Ala-Ala-Phe-Leu-Leu-Pro-Lys, indicating either a point mutation of 177 Cys → Phe or the deletion of residues 166–177. DNA sequencing of PCR-amplified DNA confirmed the former Cys → Phe substitution by indicating a point mutation of C to A at nucleotide position 5185. It appears that the aberrant electrophoretic mobility of the variant might be due to a gross conformational change associated with the formation of a new disulphide bond between Cys-168 and Cys-124. |
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Keywords: | Serum albumin Low level variant Amino acid substitution Protein electrophoresis two-dimensional Disulfide bridge |
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