Patients with phenylketonuria have light and brown hair due to reduced melanin synthesis as a result of tyrosinase inhibition. Phenylketonuria is an inherited disorder in which phenylalanine metabolism is impaired due to deficiency or reduced activity of hepatic phenylalanine hydroxylase. It is relatively common in inherited amino acid metabolism deficiency disorders. Phenylalanine (phenylalamine, PA) is an essential amino acid that is involved in the composition of various protein components, but cannot be synthesized in the human body. Under normal circumstances, about 50% of the ingested PA is used to synthesize various components of protein, and the rest is changed into tyrosine by the action of phenylalanine hydroxylase, which is then converted into dopa, dopamine, epinephrine, norepinephrine and melanin by the action of other enzymes. Phenylalanine hydroxylase is a complex enzyme system, including dihydropteroid reductase and coenzyme tetrahydrobiopterin in addition to hydroxylase itself, and any one of these enzymes can cause an increase in blood phenylalanine. When PA hydroxylase is deficient, phenylalanine, which is not involved in the first step of protein synthesis, accumulates in the plasma and is deposited in tissues throughout the body, including the brain. Phenylalanine in the blood is excreted above the renal threshold, producing phenylalaninuria. After the primary pathway of PA (hydroxylation) is blocked, the secondary metabolic pathway of PA is compensatingly hyperactive, resulting in a progressive increase in the proportion of PA converted to phenylpyruvate, phenyl lactate, n-hydroxyphenylacetic acid and phenylacetic acid. This metabolic bypass is rarely performed in normal times, so the amount of these metabolites is minimal; it is when PA hydroxylase is deficient that these metabolites reach abnormally high levels, accumulate in tissues, plasma and cerebrospinal fluid, and are excreted in large amounts in the urine, producing phenylketonuria.