A nucleotide insertion and frameshift cause albumin Kenitra, an extended and O-glycosylated mutant of human serum albumin with two additional disulfide bridges

Albumin Kenitra is a new type of genetic variant of human serum albumin tha t has been found in two members of a family of Sephardic Jews from Kenitra (Morocco). The slow-migrating variant and the normal protein were isolated by anion-exchange chromatography and, after treatment with CNBr, the digest s were analyzed by two-dimensional electrophoresis in a polyacrylamide gel. The CNBr peptides of the variant were purified by reverse-phase high perfo rmance liquid chromatography and submitted to sequence analysis. Albumin Ke nitra is peculiar because it has an elongated polypeptide chain, 601 residu es instead of 585, and its sequence is modified beginning from residue 575. DNA structural studies showed that the variant is caused by a single-base insertion, an adenine at nucleotide position 15 970 in the genomic sequence , which leads to a frameshift with the subsequent translation to the first termination codon of exon 15. Mass spectrometric analyses revealed that the four additional cysteine residues of the variant form two new S-S bridges and showed that albumin Kenitra is partially O-glycosylated by a monosialyl ated HexHexNAc structure. This oligosaccharide chain has been located to Th r596 by amino-acid sequence analysis of the tryptic fragment 592-597.