FIGLU

FIGLU, or **formiminoglutamic acid**, is an intermediate compound in the metabolic pathway of histidine, an essential amino acid in humans. It is produced during the breakdown of histidine in the liver, where the enzyme histidase converts histidine into urocanic acid, which is subsequently metabolized into formiminoglutamic acid. This process is part of the histidine catabolism pathway, which ultimately leads to the production of glutamate and formate.

FIGLU accumulates in the urine when there is a deficiency in the enzyme formiminotransferase, which is responsible for converting FIGLU into glutamic acid and formic acid. This condition can be indicative of a vitamin B2 (riboflavin) deficiency, as formiminotransferase requires riboflavin as a cofactor for its activity. Elevated levels of FIGLU in urine are often detected through laboratory tests, such as the FIGLU load test, where a histidine load is administered, and urine is subsequently analyzed for FIGLU excretion.

The FIGLU load test is occasionally used in clinical settings to assess riboflavin status, particularly in individuals suspected of having a deficiency. Symptoms of riboflavin deficiency may include skin disorders, such as cheilosis (cracking at the corners of the mouth) and angular stomatitis, as well as inflammation of the tongue (magenta tongue) and general fatigue. While FIGLU testing can provide insights into metabolic function, it is not commonly used as a routine diagnostic tool due to its limited availability and the existence of more direct assays for riboflavin levels.

In summary, FIGLU serves as a biomarker for histidine metabolism and can indicate underlying nutritional deficiencies, particularly when elevated in urine. Its clinical relevance lies in diagnosing riboflavin deficiency, though modern diagnostic approaches often prioritize more specific and accessible tests.