ALBUMIN HAWKES BAY - A LOW-LEVEL VARIANT CAUSED BY LOSS OF A SULFHYDRYL-GROUP AT POSITION-177

A slow migrating minor albumin component, representing 5% of total cir culating albumin, was detected by routine serum protein electrophoresi s and immunofixation. After treatment with 5 mM dithiothreitol the abn ormal component was found to migrate normally suggesting the attachmen t of some component to the free thiol at position 34. However, purific ation and analysis by SDS-PAGE showed that the abnormal component had a slightly lower apparent molecular weight than normal albumin. Limite d tryptic cleavage indicated the abnormal site to be in the N-terminal third of the molecule. HPLC analysis of tryptic peptides from this do main 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 mu tation of C to A at nucleotide position 5185. It appears that the aber rant electrophoretic mobility of the variant might be due to a gross c onformational change associated with the formation of a new disulphide bond between Cys-168 and Cys-124.