Acetylation of the N-terminal amino acid (-NH2 acetylation) is a common protein modification in eukaryotes but is rarely encountered in prokaryotes. In mammalians,80 to 90 percent of the cytosolic proteins are subjected to an irreversible, cotranslational amino acid acetylation at their N-terminus. Acetylation of the N-terminal amino acid (-NH2 acetylation) is a common protein modification in eukaryotes but is rarely encountered in prokaryotes. In mammalians, 80 to 90 percent of the cytosolic proteins are subjected to an irreversible, cotranslational amino acid acetylation at their N-terminus. N-acetylated proteins are catabolized in the cytosol by the ATP-ubiquitin-dependent proteasomal pathway. Several types of aminoacylases can be distinguished on the basis of substrate specificity. Aminoacylase I (ACY1; EC 3.5.1.14), the most abundant type, is a soluble homodimeric zinc binding enzyme that catalyzes the formation of free aliphatic amino acids from N-acetylated precursors. It is encoded by the aminoacylase 1 gene (ACY1) on chromosome 3p21 that comprises 15 exons (OMIM 609924). Preferred substrates of ACY1 are aliphatic amino acids with a short-chain acyl moiety, especially N-acetyl-methionine. However, ACY1 can also catalyze the reverse reaction, the synthesis of acetylated amino acids. Functional aminoacylase I is crucial in the last step in this degradation as it catalyzes the hydrolysis of N-acetylated amino acids into acetate and the free amino acid. Although N-acetylation occurs in many metabolic pathways and N-acetylated metabolites are known to accumulate in several inborn errors, such as aminoacylase I deficiency. There are only a few reports on N-acetylated amino acids detected in urine. Identification of N-acetylated amino acids by routine GC-MS may be problematic for several reasons. The major problem is linked to the identification strategy itself. Identification of an unknown compound in mass spectrometry is usually based on comparison of its spectrum against a library of reference spectra.