Hawkinsin is a sulfur amino acid identified as (2-L-cystein-S-yl-1,4-dihydroxycyclohex-5-en-1-yl)acetic acid. It is a reactive byproduct of abnormal tyrosine metabolism. Normally, the breakdown of the amino acid tyrosine involves the conversion of 4-hydroxyphenylpyruvate to homogentisate by 4-hydroxyphenylpyruvate dioxygenase. Complete deficiency of this enzyme leads to an inborn error of metabolism called tyrosinemia III. In rare cases, however, the enzyme is still able to produce the reactive intermediate 1,2-epoxyphenyl acetic acid but is unable to convert this intermediate to homogentisate (PMID: 11073718). The intermediate then spontaneously reacts with glutathione to form hawkinsin. Chronically high levels of hawkinsin are associated with hawkinsinuria (characterized by the inability to break down the amino acid tyrosine). Patients with hawkinsinuria excrete hawkinsin in their urine throughout their life. The features of this condition usually appear around the time infants are weaned off breast milk and begin to use formula. The signs and symptoms may include the following: failure to gain weight and grow at the expected rate (failure to thrive), abnormally high acid levels in the blood (acidosis), and fine or sparse hair. Hawkinsin is an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Hawkinsin is an organic acid. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart, liver, and kidney abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of untreated hawkinsinuria. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.