PHENYLKETONURIA

(PKU) also called phenylpyruvic oligophrenia hereditary inability of the body to normally metabolize the amino acid phenylalanine. Phenylalanine is normally converted in the human body to tyrosine, another amino acid, by a specific organic catalyst, or enzyme, called phenylalanine hydroxylase. This enzyme is not active in individuals who have phenylketonuria. As a result of this metabolic block, abnormally high levels of phenylalanine accumulate in the blood plasma, cerebrospinal fluid, and urine. Abnormal products of phenylalanine breakdown can also be detected in the urine. In the tissues, the excess amino acid and its abnormal metabolites interfere with various metabolic processes. The central nervous system, notably, is affected; impairment of some facet of nerve cell function manifests itself by mental retardation, epileptic seizures, and abnormal brain wave patterns, but the mechanism of injury is not known. The first behavioral signs of nerve cell damage are usually evident in an affected child from four to six months old. The retention of phenylalanine in body tissues also inhibits the course of tyrosine metabolism and leads specifically to a decrease in the formation of a product of tyrosine-melanin, the pigment found in the skin, hair, and eye. This may explain why persons with phenylketonuria generally have blond hair, blue eyes, and a fair skin. Phenylketonuria is transmitted by an autosomal recessive gene. About one person in 60 is a carrier of this gene. Statistically, two unaffected carriers of the gene can expect a 25 percent chance of having a child who is phenylketonuric, a 50 percent chance of having a child who is unaffected but is a carrier, and a 25 percent chance of having a completely normal child. Reliable tests are available to detect carriers of phenylketonuria, as well as infants who have the disorder. Approximately one in 10,000 newborn infants will show abnormally high plasma phenylalanine levels; out of these, about two-thirds will have the classic form of phenylketonuria, which, if untreated, will cause severe mental retardation. In the treatment of phenylketonuria, a diet low in phenylalanine is effective in controlling the body level of this amino acid. Such a diet, which should be maintained until adolescence, is best achieved by the total avoidance of meat, dairy, and other foods high in protein, whose intake is supplied by a special phenylalanine-free protein drink. Pregnant women who have phenylketonuria must resume the diet because the abnormally high levels of phenylalanine in their blood can severely damage an unborn child.