37 一位3個月大男嬰的血清中，有兩個化合物的濃度較高，包括苯丙胺酸（phenylalanine）與苯丙酮酸（phenylpyruvate），懷疑是苯酮尿症（phenylketonuria），但檢驗測得的苯丙胺酸羥化酶（Phenylalanine hydroxylase）活性卻正常。若下列那個酵素檢驗項目異常，仍可以支持你的苯酮尿症診斷？

酪胺酸酶（tyrosinase）

雙氫喋啶還原酵素（dihydrobiopterin reductase）

黑尿酸氧化酶（homogentisic acid oxidase）

多巴胺羥化酶（dopamine hydroxylase）

ans b

Phenylketonuria (PKU) is an autosomal recessive metabolic genetic disorder characterized by an error in the genetic code for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional.^[1]:541 This enzyme is necessary to metabolize the amino acidphenylalanine (Phe) to the amino acid tyrosine. When PAH enzymatic activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate (also known as phenylketone), which is detected in the urine.

Tetrahydrobiopterin-deficient hyperphenylalaninemia

A rarer form of hyperphenylalaninemia occurs when PAH is normal, but there is a defect in the biosynthesis or recycling of the cofactortetrahydrobiopterin (BH₄) by the patient.^[10] This cofactor is necessary for proper activity of the enzyme. The coenzyme (called biopterin) can be supplemented as treatment.

Levels of dopamine can be used to distinguish between these two types. Tetrahydrobiopterin is required to convert phenylalanine to tyrosine, but it is also required to convert tyrosine to L-DOPA (via the enzyme tyrosine hydroxylase), which in turn is converted to dopamine. Low levels of dopamine lead to high levels of prolactin. By contrast, in classical PKU, prolactin levels would be relatively normal.Tetrahydrobiopterin deficiency can be caused by defects in four different genes. These types are known as HPABH4A, HPABH4B, HPABH4C, and HPABH4D.^[11]