A Case History A two-year old child was taken to the...

A Case History A two-year old child was taken to the hospital. His mother said that he vomited frequently, especially after feedings. The child's weight and physical development were below normal. His hair, although dark, contained patches of white. A urine sample treated with ferric chloride $\left(\mathrm{FeCl}_{3}\right)$ gave a green color characteristic of the presence of phenylpyruvate. Quantitative analysis of urine samples gave the results shown in the table.(TABLE CAN'T COPY) (a) Suggest which enzyme might be deficient in this child. Propose a treatment. (b) Why does phenylalanine appear in the urine in large amounts? (c) What is the source of phenylpyruvate and phenyllactate? Why does this pathway (normally not functional) come into play when the concentration of phenylalanine rises? (d) Why does the boy's hair contain patches of white?

A Case History A two-year old child was taken to the hospital. His mother said that he vomited frequently, especially after feedings. The child's weight and physical development were below normal. His hair, although dark, contained patches of white. A urine sample treated with ferric chloride $\left(\mathrm{FeCl}_{3}\right)$ gave a green color characteristic of the presence of phenylpyruvate. Quantitative analysis of urine samples gave the results shown in the table.(TABLE CAN'T COPY) (a) Suggest which enzyme might be deficient in this child. Propose a treatment.
(b) Why does phenylalanine appear in the urine in large amounts?
(c) What is the source of phenylpyruvate and phenyllactate? Why does this pathway (normally not functional) come into play when the concentration of phenylalanine rises?
(d) Why does the boy's hair contain patches of white?

Key Concepts

Role of Tyrosine in Melanin Synthesis and Hypopigmentation

Tyrosine serves as a precursor for melanin synthesis, the pigment responsible for normal coloration of the skin, hair, and eyes. In PKU, the impaired conversion of phenylalanine to tyrosine leads to a deficiency in tyrosine, which in turn reduces melanin production. This deficiency is responsible for features such as hypopigmentation, which may manifest as patches of white hair in affected individuals.

Phenylketonuria (PKU)

PKU is an inborn error of metabolism characterized by a deficiency in the enzymatic conversion of phenylalanine to tyrosine. This genetic disorder results in an accumulation of phenylalanine and its byproducts, which can be toxic to the brain and affect overall development. Early detection and management are crucial to prevent irreversible neurological damage and other complications.

Phenylalanine Hydroxylase Deficiency

Phenylalanine hydroxylase is the enzyme responsible for converting the essential amino acid phenylalanine into tyrosine. A deficiency in this enzyme leads to elevated levels of phenylalanine, which are then shunted into alternative metabolic pathways, producing metabolites such as phenylpyruvate that are detectable in clinical urine tests. This deficiency is the primary defect in PKU.

Alternate Metabolic Pathways in Amino Acid Metabolism

When the normal pathway for metabolizing phenylalanine is compromised, excess phenylalanine is redirected into alternative routes such as transamination. This process produces byproducts like phenylpyruvate and phenyllactate, which accumulate and are excreted in the urine. These alternative pathways are typically minimal under normal conditions but become pronounced in metabolic disorders like PKU.

Dietary Management in Metabolic Disorders

Dietary management is critical in conditions such as PKU, where restricting intake of the precursor amino acid (phenylalanine) prevents the accumulation of toxic metabolites. Patients are often placed on a low-phenylalanine diet supplemented with tyrosine, since tyrosine becomes an essential amino acid in the context of phenylalanine hydroxylase deficiency. This approach helps minimize neurotoxicity and supports normal growth and cognitive development.

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