Quinolinic acid, also known as quinolinate, belongs to the class of organic compounds known as pyridinecarboxylic acids. Pyridinecarboxylic acids are compounds containing a pyridine ring bearing a carboxylic acid group. It is also classified as a pyridine-2,3-dicarboxylic acid, which is a dicarboxylic acid with a pyridine backbone. Quinolinic acid is a colorless solid. In plants, it is the biosynthetic precursor to nicotine. Quinolinic acid is found in all organisms, from microbes to plants to animals. Quinolinic acid can be biosynthesized via aspartic acid in plants. Oxidation of aspartate by the enzyme aspartate oxidase gives iminosuccinate, containing the two carboxylic acid groups that are found in quinolinic acid. Condensation of iminosuccinate with glyceraldehyde-3-phosphate, mediated by quinolinate synthase, affords quinolinic acid Quinolinic acid is also a downstream product of the kynurenine pathway, which metabolizes the amino acid tryptophan ((PMID: 22678511). The kynurenine/tryptophan degradation pathway is important for its production of the coenzyme nicotinamide adenine dinucleotide (NAD+) and produces several neuroactive intermediates including quinolinic acid, kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), and 3-hydroxyanthranilic acid (3-HANA). In animals quinolinic acid acts as an NMDA receptor agonist and has a possible role in neurodegenerative disorders (PMID: 22678511). It also acts as a neurotoxin, gliotoxin, proinflammatory mediator, and pro-oxidant molecule (PMID: 22248144). Quinolinic acid can act as an endogenous brain excitotoxin when released by activated macrophages (PMID: 15013955). Within the brain, quinolinic acid is only produced by activated microglia and macrophages. Quinolinic acid is unable to pass through the blood-brain barrier (BBB) and must be produced within the brain by microglial cells or macrophages that have passed the BBB (PMID: 22248144). While quinolinic acid cannot pass through the BBB, kynurenic acid, tryptophan and 3-hydroxykynurenine can and can subsequently act as precursors to the production of quinolinic acid in the brain (PMID: 22248144). Quinolinic acid has potent neurotoxic effects. Studies have demonstrated that quinolinic acid may be involved in many psychiatric disorders and neurodegenerative diseases in the brain including ALS, Alzheimer‚Äôs disease, brain ischemia, Parkinson‚Äôs disease, Huntington‚Äôs disease and AIDS-dementia.  Elevated CSF levels of quinolinic acid are correlated with the severity of neuropsychological deficits in patients who have AIDS. Indeed, levels of quinolinic acid in the CSF of AIDS patients suffering from AIDS-dementia can be up to twenty times higher than normal (PMID: 10936623). Quinolinic acid levels are increased in the brains of children infected with a range of bacterial infections of the central nervous system (CNS), of poliovirus patients, and of Lyme disease with CNS involvement patients. In addition, raised quinolinic acid levels have been found in traumatic CNS injury patients, patients suffering from cognitive decline with ageing, hyperammonaemia patients, hypoglycaemia patients, and systemic lupus erythematosus patients. Quinolinic acid has also been detected, but not quantified in, several different foods, such as Ceylon cinnamons, pitanga, Oregon yampahs, red bell peppers, and durians. This could make quinolinic acid a potential biomarker for the consumption of these foods.