Phenylalanine hydroxylase defect

As part of a standard neonatal screen, an infant is diagnosed with a loss of function genetic defect in the enzyme phenylalanine hydroxylase. Defects in this enzyme can result in a condition known as phenylketonuria (PKU), which results from the toxic effects of phenylalanine derived phenylketones. Fortunately, this condition can be managed by regulating the amount of phenylalanine provided in the diet. Which of the following nonessential amino acids will need to be supplied in the diet of this infant ... 

Figure28-10. The phenylalanine hydroxylase reaction. Two distinct enzymatic activities are involved. Activity II catalyzes reduction of dihydrobiopterin by NADPH, and activity I the reduction of O2 to HjO and of phenylalanine to tyrosine. This reaction is associated with several defects of phenylalanine metabolism discussed in Chapter 30.

Rarely, phenylketonuria results from a defect in the metabolism of biopterin, a cofactor for the phenylalanine hydroxylase pathway 673... 

PKU) phenylalanine hydroxylase. In care cases, defect of biopterin metabolism (Fig. 40-3 reaction 1) children. Avoidable with early institution of diet therapy. Prognosis less favorable in PKU secondary to defect of biopterin metabolism Carbidopa... 

FIGURE 40-2 The phenylalanine hydroxylase (PAH) pathway. Phenylketonuria usually is caused by a congenital deficiency of PAH (reaction 1), but it also can result from defects in the metabolism of biopterin, which is a cofactor for the hydroxylase. Enzymes (1) Phenylalanine hydroxylase (2) Dihydropteridine reductase (3) GTP cyclohydrolase (4) 6-pyruvoyltetrahydrobiopterin synthase. BH4, tetrahydrobiopterin DEDT, o-erythro-dihydroneopterin triphosphate QH2, dihydrobiopterin. 

Rarely, phenylketonuria results from a defect in the metabolism of biopterin, a cofactor for the phenylalanine hydroxylase pathway. The electron donor for phenylalanine hydroxylase is tetrahydrobiopterin (BH4), which transfers electrons to molecular oxygen to form tyrosine and dihydrobiopterin (QH2 Fig. 40-2 reaction 2). BH4 is regenerated from QH2 in an NADH-dependent reaction that is catalyzed by dihydropteridine reductase (DHPR), which is widely distributed. In the brain, this... 

A much more serious genetic disease, first described by Foiling in 1934, is phenylketonuria. Here the disturbance in phenylalanine metabolism is due to an autosomal recessive deficiency in liver phenylalanine hydroxylase (Jervis, 1954) which normally converts significant amounts of phenylalanine to tyrosine. Phenylalanine can therefore only be metabolized to phenylpyruvate and other derivatives, a route which is inadequate to dispose of all the phenylalanine in the diet. The amino acid and phenylpyruvate therefore accummulate. The condition is characterized by serious mental retardation, for reasons which are unknown. By the early 1950s it was found that if the condition is diagnosed at birth and amounts of phenylalanine in the diet immediately and permamently reduced, mental retardation can be minimized. The defect is shown only in liver and is not detectable in amniotic fluid cells nor in fibroblasts. A very sensitive bacterial assay has therefore been developed for routine screening of phenylalanine levels in body fluids in newborn babies. 

Phenylalanine hydroxylase (PH) which requires tetrahydrobiopterin (BH4) as a cofactor, is defective in cases of phenylketonuria (PKU). This is a rare (prevalence 1 / 15 000 in the United Kingdom) genetic condition characterized by fair complexion, learning difficulties and mental impairment. If PH is either not present in the hepatocytes or is unable to bind BH4 and is therefore non functional, phenylalanine accumulates within the cells. Enzymes in minor pathways which are normally not very active metabolize phenylalanine ultimately to phenylpyruvate (i.e. a phenylketone). To use the traffic flow analogy introduced in Chapter 1, the main road is blocked so vehicles are forced along side roads. Phenylpyruvate is excreted in the urine (phenyl-ketone-uria), where it may be detected but a confirmatory blood test is required for a reliable diagnosis of PKU to be made. 

T Given that many amino acids are either neurotransmitters or precursors or antagonists of neutrotransmitters, genetic defects of amino acid metabolism can cause defective neural development and mental retardation. In most such diseases specific intermediates accumulate. For example, a genetic defect in phenylalanine hydroxylase, the first enzyme in the catabolic pathway for phenylalanine (Fig. 18-23), is responsible for the disease phenylketonuria (PKU), the most common cause of elevated levels of phenylalanine (hyperphenylalaninemia). 

Tyrosine is normally a nonessential amino acid, but individuals with a genetic defect in phenylalanine hydroxylase require tyrosine in their diet for normal growth. Explain. 

The parents are both heterozygous for the phenylalanine hydroxylase gene. They, thus, have both fragment "a" (normal) and fragment "b (defective) cleaved by a restriction nuclease. 

This child is affected (lacks phenylalanine hydroxylase activity) and shows only fragment "b" when DNA is digested with the same restriction endonuclease. Thus, the defective gene is associated with the polymorphism giving fragment "b."... 

The molecular defect in the phenylalanine hydroxylase (PAH) gene in the family is not known. The family... 

The hereditary absence of phenylalanine hydroxylase, which is found principally in the liver, is the cause of the biochemical defect phenylketonuria (Chapter 25, Section B).430 4308 Especially important in the metabolism of the brain are tyrosine hydroxylase, which converts tyrosine to 3,4-dihydroxyphenylalanine, the rate-limiting step in biosynthesis of the catecholamines (Chapter 25), and tryptophan hydroxylase, which catalyzes formation of 5-hydroxytryptophan, the first step in synthesis of the neurotransmitter 5-hydroxytryptamine (Chapter 25). All three of the pterin-dependent hydroxylases are under complex regulatory control.431 432 For example, tyrosine hydroxylase is acted on by at least four kinases with phosphorylation occurring at several sites.431 433 4338 The kinases are responsive to nerve growth factor and epidermal growth factor,434 cAMP,435 Ca2+ + calmodulin, and Ca2+ + phospholipid (protein kinase C).436 The hydroxylase is inhibited by its endproducts, the catecholamines,435 and its activity is also affected by the availability of tetrahydrobiopterin.436... 

Figure 19-4. Differences between the frequency of phenylketonuria (PKU) and hyper-phenylalaninemia (HPA) caused by problems in tetrahydrobiopterin (BH4) metabolism and reports of positive therapeutic responses to BH4 therapy. (A) Frequency of PKU and HPA cases documented to be caused by defects in BH4 metabolism. D, Clinical cases documented to be caused by defects in BH4 D, clinical cases presumable due to a defect in the phenylalanine hydroxylase enzyme. (II) Frequency of PKU and HPA cases that have been reported to respond positively to BH4 therapy. , Positive response to BH4 therapy 0, no response to BH4 therapy. Different reports in the literature varied with respect to the numbers of individuals responding to BH4 therapy. The differences in reported numbers of BH4 responders are indicated by the boxes with cross-hatching.

Erlandsen H, PeyAL, Gamez A, et al. Correction of kinetic and stability defects by tetrahydrobiopterin in phenylketonuria patients with certain phenylalanine hydroxylase mutations. Proc. Natl. Acad. Sci. (USA) 101 16903-16908,... 

This therapeutic approach is based on the concept that the function of certain defective phenylalanine hydroxylases can be bolstered by increased amounts of the cognate coenzyme tetrahydrobiopterin. In fact, the relatively large number of patients with classic phenylketonuria may provide an economic incentive for the development of a biotechnological process for the bulk production of the coenzyme. 

C-7) Phenylketonuria (deficiency of phenylalanine hydroxylase). Occasionally, the defect is not in the enzyme but in the ability to regenerate tetrahydrobiop-terin, which is also necessary for the reaction. There is a buildup and excretion of phenylpyruvate in the urine, giving it a mousy odor. Mental retardation is a prominent feature. Diagnosis can be made by routine urine testing for phenylpyruvate or serum testing for elevated phenylalanine levels. The condition is treated with a diet low in phenylalanine. Sometimes, tetrahydrobiopterin deficiency may be treated by supplying biopterin,... 

PKU is a serious lEM caused by a liver enzyme deficiency. In PKU, phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine, is defective. Several different mutations are responsible for altering or reducing the activity of the phenylalanine hydroxylase gene. Because PKU patients cannot make the pigment melanin, 90% of PKU patients are blond-haired with blue eyes. Other clinical features include seizures, a mousy body odor, and eczema. Left untreated, accumu-... 

Thousands of diseases related to deficient or defective enzymes occur, many of which are rare. For example, in phenylketonuria (which has an incidence of 1 in 10,000 births in whites and Asians), the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine, is deficient. Phenylalanine accumulates, and tyrosine becomes an essential amino acid that is required in the diet. Mental retardation is a result of metabolic derangement. A more common problem is lactase deficiency, which occurs in 69% to 90% of American Indians, blacks, and Asians, and in 10% of whites. Lactose is not digested normally and accumulates in the gut where it is metabolized by bacteria. Bloating, abdominal cramps, and watery diarrhea result. 

B. PKU is caused by a deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine. A defect in tyrosine degradation causes homogentisic acid to accumulate and produce dark pigments (alcaptonuria). A defect in the conversion of tyrosine to the skin pigment melanin causes albinism. 

Phenylketonuria (PKU) is an inborn error of metabolism by which the body is unable to convert surplus phenylalanine (PA) to tyrosine for use in the biosynthesis of, for example, thyroxine, adrenaline and noradrenaline. This results from a deficiency in the liver enzyme phenylalanine 4-mono-oxygenase (phenylalanine hydroxylase). A secondary metabolic pathway comes into play in which there is a transamination reaction between PA and a-keto-glutaric acid to produce phenylpyruvic acid (PPVA), a ketone and glutamic acid. Overall, PKU may be defined as a genetic defect in PA metabolism such that there are elevated levels of both PA and PPVA in blood and excessive excretion of PPVA (Fig. 25.7). 

Abnormal indole derivatives in the urine and low levels of serotonin (a product of tryptophan metabolism) in blood and brain point to a defect in tryptophan metabolism in PKU. 5-Hydroxytryptophan decarboxylase, which catalyzes the conversion of 5-hydroxytryptophan to serotonin, is inhibited in vitro by some of the metabolites of phenylalanine. Phenylalanine hydroxylase is similar to the enzyme that catalyzes the hydroxylation of tryptophan to 5-hydroxytryptophan, a precursor of serotonin. In vitro, phenylalanine is also found to inhibit the hydroxylation of tryptophan. The mental defects associated with PKU may be caused by decreased production of serotonin. High phenylalanine levels may disturb the transport of amino... 

The answer is c. (Muiray, pp 323-346. Scrivei pp 1667-1724. Sack, pp 121-144. Wilson, pp 287-324.) Decreased melanin can occur in PKU because melanin is produced from phenylalanine and tyrosine. The defect in most children with PKU is deficiency of phenylalanine hydroxylase. Rare children have deficiency of biopterin cofactor due to a defect in its synthetic enzyme that is also autosomal recessive. Phenylalanine is converted to tyrosine by phenylalanine hydroxylase, so deficient tyrosine can occur in children on restrictive diets. 

The provision of reducing equivalents to phenylalanine hydroxylase is dependent on reduction of dihydrobiopterin by NADH catalyzed by the enzyme dihydropteridine reductase, as shown in Figure 38-2. This reduction is dependent on the availability of biopterin and therefore on the biopterin synthetic pathway. Thus any genetic or protein folding defect in either dihydropteridine reductase or the biopterin biosynthetic enzymes would compromise the efficacy of phenylalanine hydroxylation to tyrosine resulting in hyperphenylalaninemia and also phenylketonuria resulting from inaease transamination of phenylalanine to phenylpyruvate. 

Regulation of the dietary intake of amino acids can also be important when considering the treatment of certain defects in amino add biosynthesis. Phenylalanine is an essential amino acid that is also used to generate the nonessential amino add tyrosine. The enzyme that carries out this readion is the mixed function oxidase phenylalanine hydroxylase (PAH). Inherited deficiencies in PAH are associated with a condition known as phenylketonuria (PKU see Case 38). The absence of PAH results in elevations of phenylalanine and various phenylketones, the accumulation of which is associated with the neurologic defects seen in this disorder. PKU can be treated by controlling the dietary intake of phenylalanine. Diets low in phenylalanine will help prevent excessive elevations in phenylalanine. Phenylalanine can not be completely eliminated from the diet because it is an essential amino acid needed for protein synthesis. In the absence of PAH activity, tyrosine becomes an essential amino add because it cannot be generated from phenylalanine. 

Conservation of amino acids filtered at the glomerulus is made possible by the existence of four main transport systems for specific amino acids that facilitate active reabsorption of these amino acids from the proximal tubule. A lack or deficiency of the transport system responsible for the absorption of valine, alanine, cystine, and tryptophan, and of the transport system for arginine, lysine, cystine, and ornithine, leads to excretion of these specific amino acids in urine, which is characterized as renal aminoaciduria to distinguish it from overflow aminoaciduria. In the latter situation, the production of amino acids far exceeds the proximal tubular reabsorption capacity, thus leading to overflow of amino acids into urine. This can occur due to defective metabolism of amino acids, as is the case when phenylalanine cannot be metabolized due to the deficiency of the enzyme phenylalanine hydroxylase, or to the inability to deaminate amino acids in liver disease. 

PKU can be caused by deficiencies in phenylalanine hydroxylase and by enzymes catalyzing the formation and regeneration of 5,6,7,8-tetrahydrobiopterin. How can this second defect cause the symptoms of PKU ... 

Figure 1.2 Consequences of a metabolic block in pheylalanine-tyrosine Defective phenylalanine hydroxylase can lead to the accumulation of phenylalanine, which can cause damage to brain cells and mental retardation in phenylketonuric babies. Another metabolic blockage caused by a defective enzyme can lead to alcaptonuria.

Phenylalanine hydroxylase uses teirahydrobioptcrin (BH4) as a cofactor. Defective BH4 supply or regeneration, due to deficiency of dihydropteridine reductase, have been identified as rare cau.ses of hyperphenylalaninaemia. a... 

The hereditary absence of phenylalanine hydroxylase, which is found principally in the liver, is the cause of the biochemical defect phenylketonuria (Chapter 25, Section Especially important in the me-... 

A small subset of patients with hyperphenylalaninemia show an appropriate reduction in plasma phenylalanine levels with dietary restriction of this amino acid however, these patients still develop progressive neurologic symptoms and seizures and usually die within the first 2 years of life ("malignant" hyperphenylalaninemia). These infants exhibit normal phenylalanine hydroxylase (PAH) activity but have a deficiency in dihy-dropteridine reductase (DHPR), an enzyme required for the regeneration of tetrahydro-biopterin (BH4), a cofactor of PAH (see Fig. 39.18). Less frequently, DHPR activity is normal but a defect in the biosynthesis of BH4 exists. In either case, dietary therapy corrects the hyperphenylalaninemia. However, BH4 is also a cofactor for two other hydroxy-lations required in the synthesis of neurotransmitters in the brain the hydroxylation of tryptophan to 5-hydroxytryptophan and of tyrosine to L-dopa (see Chapter 48). It has been suggested that the resulting deficit in central nervous system neurotransmitter activity is, at least in part, responsible for the neurologic manifestations and eventual death of these patients. 

The biochemical defect in classical PKU is the inability to carry out the normal hydroxylation of L-phenylalanine to tyrosine. The enzyme catalyzing this reaction is a so-called mixed function oxidase, phenylalanine hydroxylase, which is localized only to the liver, kidney, and pancreas. Dietary phenylalanine and phenylalanine produced... 

These patients can be diagnosed by administration of tetrahydrobiopterin, which will cause a fall in the plasma phenylalanine level. Evidence thus far marshaled seems to support a defect in biosynthesis of biopterin, the precursor to tetrahydrobiopterin and the active cofactor for phenylalanine hydroxylase (Kure... 

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