Classification of an Alloalbumin

Albumin is the most abundant of the plasma proteins and has an important role in the maintenance of the oncotic pressure of the extracellular fluid and as a transporter of drugs, hormones and other molecules around the body. It is, however, not essential for life as several cases of analbuminaemia where there is an almost total lack of circulating albumin have been identified. 1 Over 100 albumin variants, based mainly on their electrophoretic mobility, have been described. These anomalies include analbuminaemia, the temporary appearance of oligomeric albumins formed during storage, the bisalbumins, and the genetically distinct albumin variants or the alloalbumins.i Nearly all alloalbumins are discovered as incidental findings during routine investigations and, while occasionally they may pose some technical problems in elucidating the nature of anomalous hormone or drug levels, they are rarely associated with any clinical manifestations. Albumin is a single polypeptide chain containing 585 amino acids with a molecular weight of approximately 66000 Da. The majority of the genetic albumin variants are the result of single point mutations in the primary gene sequence causing the substitution of one amino acid in the polypeptide chain, although translational defects such as failure of N-terminal signal peptide cleavage and premature chain termination at the C-terminal also contribute to the genetic variants.! However, only mutations which alter the charge on the albumin molecule will be recognized by electrophoretic techniques and some genetic variants may remain unidentified as silent alleles. In the past, when an alloalbumin was detected it was classified according to its migration in conventional alkaline pH