N-Acetyl-L-cysteine (NAC) or N-Acetylcysteine is the N-acetyl derivative of the amino acid L-cysteine and is a precursor in the formation of the antioxidant glutathione in the body. N-Acetylcysteine, 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-Acetyl-L-cysteine can also be classified as an alpha amino acid or a derivatized alpha amino acid. Technically, N-Acetyl-L-cysteine is a biologically available N-terminal capped form of the proteinogenic alpha amino acid L-cysteine. 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-acetylcysteine 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 cysteine can also occur. The enzyme known as cysteine-S-conjugate N-acetyltransferase (EC 2.3.1.80) catalyzes the transfer of the acetyl group of acetyl CoA to the amino group of cysteine. This enzyme is an important participant in glutathione metabolism and the production of glutathione. The thiol (sulfhydryl) group in N-Acetylcysteine confers antioxidant effects and is able to reduce free radicals. N-Acetylcysteine is a pharmacological agent used in the management of paracetamol (acetaminophen) overdoses. When acetaminophen is taken in large quantities, a minor metabolite called N-acetyl-p-benzoquinone imine (NAPQI) accumulates within the body. NAPQI is normally conjugated by glutathione, but when taken in excess, the body's glutathione reserves are not sufficient to deactivate the toxic NAPQI. In the treatment of acetaminophen overdose, N-acetylcysteine acts to maintain or replenish depleted glutathione reserves in the liver and enhance non-toxic metabolism of acetaminophen. These actions serve to protect liver cells from NAPQI toxicity. For this particular indication, N-acetylcysteine is available under the trade names Mucomyst (Bristol-Myers Squibb) and Parvolex (GSK). N-Acetylcysteine is also used as a mucolytic agent to reduce the viscosity of mucous secretions. It has also been shown to have antiviral effects in patients with HIV due to inhibition of viral stimulation by reactive oxygen intermediates. Acetylcysteine has been studied for a number of psychiatric disorders. There is tentative evidence for N-acetylcysteine being useful in the treatment of Alzheimer's disease, autism, bipolar disorder, drug-induced neuropathy, major depressive disorder, obsessive-compulsive disorder, and schizophrenia (PMID: 21118657). Many N-acetylamino acids, including N-acetylcysteine, can act as uremic toxins if present in high abundance in the serum or plasma (PMID: 21970774; 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).