a)Individuals diagnosed with PKU are placed on a low phenylalanine diet. Why wou

Question

a)Individuals diagnosed with PKU are placed on a low phenylalanine diet. Why would a phenylalanine-free diet be undesirable?

b)Why should patients with PKU avoid the artificial sweetener aspartame?

c)Explain why individuals on a low-phenylalanine diet may need to increase their tyrosine intake?

Phenylketonuria is an inherited disease which results from the lack of the enzyme phenylalanine shown in Figure 30.1. In the phenylketonuric patient, phenylalanine accumulates which is eventually blood and urine and has the effect of causing mental retardation if untreated. Screening programs identify PKU babies at birth, and treatment consists of a low phenylalanine diet until maturation of the brain is completed. The structure and biochemical properties of the PAH enzyme have been well-studied The gene for PAH has been isolated and has been localized to chromosome 12. The PAH enzyme is a protein 451 amino acids in length with a molecular weight of 51,900 daltons. More than 60 different mutant genes giving rise to nonfunctional PAH proteins have been identified in PKU patients.

Explanation / Answer

Since patients with PKU cannot metabolize phenylalanine, protein rich diet is not recommended for them. Phenyalanine is an aminoacid present in proteins and also in artificial sweetners such as aspartame, thus a low protein diest is recommended (if the intake is too low this can also result in high blood phenylalanine concentrations) and aspartame is avoided. There are reports of deficiencies such as vitamin B12, selenium, etc. Bone density is also diminished in these patients, although whether this is due to the disease itself or to the diet is still to be determined.

Usually phenylalanine is transformed into tyrosine within the body in healthy individuals.Taking that as a fact, the amount of tyrosine in the amino acid supplement should be the amount of phenylalanine not taken and the normal amount of tyrosine.

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