The molecular basis of aminoacylase 1 deficiency
Résumé
Aminoacylase 1 (ACY1) is a zinc-binding enzyme which hydrolyzes -acetyl amino acids into the free amino acid and acetic acid. Deficiency of ACY1 due to mutations in the gene follows an autosomal-recessive trait of inheritance and is characterized by accumulation of -acetyl amino acids in the urine. In affected individuals neurological findings such as febrile seizures, delay of psychomotor development and moderate mental retardation have been reported. Except for one missense mutation which has been studied in , mutations underlying ACY1 deficiency have not been characterized so far. This has prompted us to approach expression studies of all mutations known to occur in ACY1 deficient individuals in a human cell line (HEK293), thus providing the authentic human machinery for posttranslational modifications. Mutations were inserted using site directed mutagenesis and ACY1 enzyme activity was assessed in cells overexpressing ACY1, using mainly the natural high affinity substrate -acetyl methionine. Overexpression of the wild type enzyme in HEK293 cells resulted in an approximately 50-fold increase of the ACY1 activity of homogenized cells. Most mutations resulted in a nearly complete loss of enzyme function. Notably, the two newly discovered mutations p.Arg378Trp, p.Arg378Gln and the mutation p.Arg393His yielded considerable residual activity of the enzyme, which is tentatively explained by their intramolecular localization and molecular characteristics. In contrast to ACY1 variants which showed no detectable ACY1 activity, ACY1 proteins with the mutations p.Arg378Trp, p.Arg378Gln and p.Arg393His were also detected in Western blot analysis. Investigations of the molecular bases of additional cases of ACY1 deficiency contribute to a better understanding of this inborn error of metabolism whose clinical significance and long-term consequences remain to be elucidated.
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