N-Acetyl-L-methionine or N-Acetylmethionine, belongs to the class of organic compounds known as N-acyl-alpha amino acids. N-acyl-alpha amino acids are compounds containing an alpha amino acid which bears an acyl group at its terminal nitrogen atom. N-Acetylmethionine can also be classified as an alpha amino acid or a derivatized alpha amino acid. Technically, N-Acetylmethionine is a biologically available N-terminal capped form of the proteinogenic alpha amino acid L-methionine. N-acetyl amino acids can be produced either via direct synthesis of specific N-acetyltransferases or via the proteolytic degradation of N-acetylated proteins by specific hydrolases. N-terminal acetylation of proteins is a widespread and highly conserved process in eukaryotes that is involved in protection and stability of proteins (PMID: 16465618).  About 85% of all human proteins and 68% of all yeast proteins are acetylated at their N-terminus (PMID: 21750686). Several proteins from prokaryotes and archaea are also modified by N-terminal acetylation. The majority of eukaryotic N-terminal-acetylation reactions occur through N-acetyltransferase enzymes or NAT‚Äôs (PMID: 30054468).  These enzymes consist of three main oligomeric complexes NatA, NatB, and NatC, which are composed of at least a unique catalytic subunit and one unique ribosomal anchor. The substrate specificities of different NAT enzymes are mainly determined by the identities of the first two N-terminal residues of the target protein. The human NatA complex co-translationally acetylates N-termini that bear a small amino acid (A, S, T, C, and occasionally V and G) (PMID: 30054468). NatA also exists in a monomeric state and can post-translationally acetylate acidic N-termini residues (D-, E-). NatB and NatC acetylate N-terminal methionine with further specificity determined by the identity of the second amino acid.  N-acetylated amino acids, such as N-acetylmethionine can be released by an N-acylpeptide hydrolase from peptides generated by proteolytic degradation (PMID: 16465618). In addition to the NAT enzymes and protein-based acetylation, N-acetylation of free methionine can also occur. In particular, N-Acetylmethionine can be biosynthesized from L-methionine and acetyl-CoA by the enzyme methionine N-acetyltransferase (EC 2.3.1.66). Excessive amounts N-acetyl amino acids including N-acetylmethionine (as well as N-acetylglycine, N-acetylserine, N-acetylglutamine, N-acetylglutamate, N-acetylalanine, N-acetylleucine and smaller amounts of N-acetylthreonine, N-acetylisoleucine, and N-acetylvaline) can be detected in the urine with individuals with acylase I deficiency, a genetic disorder (PMID: 16465618). Aminoacylase I is a soluble homodimeric zinc binding enzyme that catalyzes the formation of free aliphatic amino acids from N-acetylated precursors. In humans, Aminoacylase I is encoded by the aminoacylase 1 gene (ACY1) on chromosome 3p21 that consists of 15 exons (OMIM 609924). Individuals with aminoacylase I deficiency will experience convulsions, hearing loss and difficulty feeding (PMID: 16465618). ACY1 can also catalyze the reverse reaction, the synthesis of acetylated amino acids. Many N-acetylamino acids, including N-acetylmethionine are classified as uremic toxins if present in high abundance in the serum or plasma (PMID: 26317986; PMID: 20613759). Uremic toxins are a diverse group of endogenously produced molecules that, if not properly cleared or eliminated by the kidneys, can cause kidney damage, cardiovascular disease and neurological deficits (PMID: 18287557).