What amino acid is primarily involved in the metabolic disorder known as homocystinuria?

Homocystinuria is a metabolic disorder characterized by an accumulation of homocysteine, an amino acid that is not typically found in high concentrations in the body. In this condition, there is a defect in the enzymes responsible for the metabolism of homocysteine, leading to elevated levels in the blood and urine.

Homocysteine is produced from the amino acid methionine through a series of enzymatic reactions. In people with homocystinuria, various enzyme deficiencies (such as cystathionine beta-synthase or deficiency of vitamin B6, B12, or folate) impede the conversion of homocysteine into cysteine or its re-methylation to methionine.

Elevated homocysteine levels can lead to serious health issues, including cardiovascular problems, skeletal abnormalities, and neurological complications. Thus, the primary amino acid directly involved in this disorder is homocyst(e)ine, which is a key marker for diagnosing homocystinuria and understanding its pathophysiology.