Specialized Products of Amino Acids Phenylalanine, Tyrosine
Phenylalanine
Phenylalanine is first converted to tyrosine. Hyperphenylalaninemias arise from defects in phenylalanine hydroxylase itself (type I, classic phenylketonuria or PKU), in dihydrobiopterin reductase (types II and III), or in dihydrobiopterin biosynthesis (types IV and V). DNA probes facilitate prenatal diagnosis of defects in phenylalanine hydroxylase or dihydrobiopterin reductase. A diet low in phenylalanine can prevent the mental retardation of PKU (frequency 1:10,000 births). Elevated blood phenylalanine may be detectable by a less reliable screening test that employs FeCl3 to detect urinary phenylpyruvate. FeCl3 screening for PKU of the urine of newborn infants is compulsory in the United States and many other countries.
Tyrosine
The probable metabolic defect in type I tyrosinemia (tyrosinosis) is at fumarylacetoacetate hydrolase. Therapy employs a diet low in tyrosine and phenylalanine. Untreated acute and chronic tyrosinosis leads to death from liver failure. Alternate metabolites of tyrosine are also excreted in type II tyrosinemia (Richner-Hanhart syndrome), a defect in tyrosine aminotransferase, and in neonatal tyrosinemia, due to lowered p-hydroxyphenylpyruvate hydroxylase activity. Therapy employs a diet low in protein.
