ENZYME DEFICIENCIES: PHENYLKETONURIA AND GALACTOSEMIA
More than a hundred enzyme deficiencies have been identified. These are generally inherited and are sometimes called inborn errors of metabolism. Many of them are evident shortly after birth and others are acquired later in life.
Phenylketonuria
Clinical findings. Phenylketonuria occurs in about 1 infant of every 10,000. The infant is born without the enzyme necessary to use phenylalanine, one of the essential amino acids. As a result the level of the blood phenylalanine is increased and phenylketones are also excreted in the urine – hence the name phenylketonuria (PKU). The deficiency can be diagnosed shortly after birth by blood and urine tests.
High blood levels of phenylalanine are toxic. The infants are usually blond, blue-eyed, fair, and often have eczema. Untreated infants are hyperactive, irritable, and have an unpleasant personality. They have a persistent musty or gamey body odor caused by the production of the phenylketones. If the disease is not diagnosed in the first months of life, mental retardation is usually severe.
Dietary management. Phenylketonuria is successfully treated by a phenyla-lanine-restricted diet when diagnosis is made in the first months of life. If treatment is delayed, the mental retardation that has occurred cannot be reversed.
The phenylalanine is adjusted to maintain a normal level in the blood. Since this is an essential amino acid, some must be provided to meet normal growth needs. This has been estimated to be 20 to 30 mg per kg. Thus, a 3-month infant weighing 6 kg would need 120 to 180 mg phenylalanine daily.
Proteins contain about 5 per cent phenylalanine. The recommended allowance for protein for a 6 kg infant is about 13 gm. From milk formulas and supplementary foods this would supply 650 mg phenylalanine, which is about 4 to 5 times as much as he should have.
A special formula Lofenalac is used for these infants. It supplies just enough phenylalanine for growth and is adequate for all other nutrients. As the infant grows, ordinary foods that are low in phenylalanine are added. Mothers are given detailed food lists for fruits, vegetables, breads, and cereals from which to choose, and careful instructions are needed for measurement. Even low-protein foods such as fruits contain some phenylalanine and must be given in measured amounts.
Successful treatment depends upon dietary adjustment at frequent intervals-
Based upon the blood levels of phenylalanine the diet is adjusted so that the amount furnished to the body is just sufficient for growth needs. As the child grows the diet is increased to meet all nutritional requirements. The low-phenylalanine diet is continued throughout the early years of childhood. Whether it needs to be continued indefinitely is not yet known.
Galactosemia
Clinical findings. From birth infants with this condition lack transferase, a liver enzyme that converts galactose to glucose. Galactose is one of the simple sugars resulting when lactose is digested. When the enzyme is absent the levels of galactose in the blood reach toxic levels. A few days after birth the infant has vomiting, diarrhea, drowsiness, edema, liver failure, and hemorrhage. If the infant survives, mental retardation is severe.
Dietary management. A galactose-free formula must be started within the first few days of life. Nutramigen, ProSobee, Mul-Soy, and MBF (Meat-Base Formula) are suitable. Since milk is the only food that supplies lactose, other foods are added to the infant’s diet as he grows. All milk-containing foods must be rigidly excluded.
*141/234/5*
GENERAL HEALTH.
