Regulation of albumin synthesis

During the past few years, much has been learned about the basic biosynthesis of serum albumin. This protein, which now occupies so much of the liver’s productive capacity in terms of an exported protein, exists intracellularly in two forms, pro-albumin and albumin. The latter is identical to serum albumin, while the former is an albumin molecule containing either a pentapeptide or hexapeptide attached to the amino terminus of albumin (1,2). This small peptide moiety is removed by an intracellular protease at some time following synthesis and just prior to secretion (3). More recently, it has been demonstrated that the initial albumin molecule synthesized contains eighteen additional amino acids attached to the small peptide moiety of proalbumin (4). It may well be that this peptide extension on proalbumin is the signal peptide necessary for the interaction of the large ribosomal subunit with the endoplasmic reticulum allowing for transfer of the nascent albumin chain across the endoplasmic membrane with subsequent proteolysis of the signal peptide to form the proalbumin (5). It is of interest that the amino acid sequence in the peptide extension of the pre and pro albumin molecule is quite equivalent to sequences in other precursor molecules for proteins whose eventual destiny is export from some other cellular organ (6).