title: Accumulation of altered aspartyl residues in erythrocyte proteins from patients with Down's syndrome.
abstract: Spontaneous protein deamidation of labile Asn residues, generating L-isoaspartates and D-aspartates, is associated with cell aging and is enhanced by an oxidative microenvironment; to minimize the damage, the isoaspartate residues can be 'repaired' by a specific L-isoaspartate (D-aspartate) protein O-methyltransferase (PIMT). As both premature aging and chronic oxidative stress are typical features of Down's syndrome (DS), we tested the hypothesis that deamidated proteins may build up in trisomic patients. Blood samples were obtained from children with karyotypically confirmed full trisomy 21 and from age-matched healthy controls. Using recombinant PIMT as a probe, we demonstrated a dramatic rise of L-isoaspartates in erythrocyte membrane proteins from DS patients. The content of D-aspartate was also significantly increased. The integrity of the repair system was checked by evaluating methionine transport, PIMT specific activity, and intracellular concentrations of adenosylmethionine and adenosylhomocysteine. The overall methylation pathway was directly monitored by incubating fresh red blood cells with methyl-labeled methionine; a three-fold increase of protein methyl esters was detected in trisomic children. Deamidated species include ankyrin, band 4.1, band 4.2 and the integral membrane protein band 3; ankyrin and band 4.1 were significantly hypermethylated in DS. When DS red blood cells were subjected to oxidative treatment in vitro, the increase of protein deamidation paralleled lipid peroxidation and free radical generation. We observed a similar pattern in Epstein-Barr virus B-lymphocytes from trisomic patients. In conclusion, our findings support the hypothesis that protein instability at asparagine sites is a biochemical feature of DS, presumably depending upon the oxidative microenvironment. The possible pathophysiological implications are discussed.
authors: Department of Biochemistry and Biophysics, School of Medicine, Second University of Naples, Italy.
authors: Patrizia
authors: P
authors: Galletti
authors: Maria Luigia
authors: ML
authors: De Bonis
authors: Alvara
authors: A
authors: Sorrentino
authors: Marianna
authors: M
authors: Raimo
authors: Stefania
authors: S
authors: D'Angelo
authors: Iris
authors: I
authors: Scala
authors: Generoso
authors: G
authors: Andria
authors: Antimo
authors: A
authors: D'Aniello
authors: Diego
authors: D
authors: Ingrosso
authors: Vincenzo
authors: V
authors: Zappia
mesh_terms: D016022:Case-Control Studies; D002648:Child; D004314:Down Syndrome / Q000097:blood* / Q000473:pathology; D004905:Erythrocyte Aging; D004910:Erythrocyte Membrane / Q000378:metabolism*; D004854:Herpesvirus 4, Human / Q000502:physiology; D006801:Humans; D026581:Isoaspartic Acid / Q000378:metabolism*; D008214:Lymphocytes / Q000166:cytology / Q000378:metabolism / Q000821:virology; D008565:Membrane Proteins / Q000378:metabolism*; D008706:Methemoglobin / Q000378:metabolism; D008715:Methionine / Q000378:metabolism; D008745:Methylation; D018384:Oxidative Stress; D026601:Protein D-Aspartate-L-Isoaspartate Methyltransferase / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012435:S-Adenosylhomocysteine / Q000378:metabolism
chemical_list: D026581:Isoaspartic Acid; D008565:Membrane Proteins; D011994:Recombinant Proteins; D008706:Methemoglobin; D012435:S-Adenosylhomocysteine; D008715:Methionine; D026601:Protein D-Aspartate-L-Isoaspartate Methyltransferase