phenylketonuria

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Phenylketonuria

Phenylketonuria (PKU) is the most common metabolic disorder in amino acid metabolism.. The incidence of PKU is 1 in 10.000 births. It is due to the deficiency of the hepatic enzyme, phenylalanine hydroxylase. caused by an autosomal recessive gene. In recent years, a variant of PKU-due to a defect in dihydrobiopterin reductase (relatively less)-has been reported. This enzyme deficiency impairs the synthesis of tetrahydrobiopterin required for the action of phenylalanine hydroxylase  The net outcome in PKU is that
phenylalanine is not converted to tyrosine. 

Phenyl alanine metabolism in PKU :-
Phenylketonuria primarily causes the accumula- tion of phenylalanine in tissues and blood, and results in its increased excretion in urine. Due to disturbances in the routine metabolism, phenyl. alternate pathways resulting in the excessive production alanine is diverted to of phenylpyruvate, phenylacetate, phenyllactate and phenylacetylglutamine. All these metabolites are excreted in urine in high concentration in PKU. Phenylacetate gives the urine a mousy odour.The name phenylketonuria is coined due to the fact that the metabolite phenylpyruvate i keto acid (C6H3CH2-CO-COO) excreted in urine in high amounts.

Clinical/biochemical manifestations of PKU: The disturbed metabolism of phenylalanine- resulting in the increased concentration of phenylalanine and its metabolites in the body- causes many clinical and biochemical manifestation.

1. Effects on central nervous system: Mental retardation, failure to walk or talk, failure of growth, seizures and tremor are the characteristic findings in PKU. If untreated, the patients show very low IQ (below 50). The biochemical basis of mental retardation in PKU is not well understood. There are however many explanations are offered.

* Accumulation of phenylalanine in brain impairs the transport and metabolism of other aromatic amino acids (tryptophan and tyrosine).

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