Phenylpyruvate accumulation

In individuals with PKU, a secondary, normally little-used pathway of phenylalanine metabolism comes into play. In this pathway phenylalanine undergoes transamination with pyruvate to yield phenylpyruvate (Fig. 18-25). Phenylalanine and phenylpyruvate accumulate in the blood and tissues and are excreted in the urine—hence the name phenylketonuria. Much of the phenylpyruvate, rather than being excreted as such, is either decarboxylated to phenylacetate or reduced to phenyllactate. Phenylacetate imparts a characteristic odor to the urine, which nurses have traditionally used to detect PKU in infants. The accumulation of phenylalanine or its metabolites in early life impairs normal development of the brain, causing severe mental retardation. This may be caused by excess phenylalanine competing with other amino acids for transport across the blood-brain barrier, resulting in a deficit of required metabolites. 

FIGURE 18-25 Alternative pathways for catabolism of phenylalanine in phenylketonuria. In PKU, phenylpyruvate accumulates in the tissues, blood, and urine. The urine may also contain phenylacetate and phenyllactate. 

A genetic disease is the result of a defective enzyme caused by a mutation in its genetic code. For example, phenylketonuria (PKU) results when DNA cannot direct the synthesis of the enzyme phenylalanine hydroxylase, required for the conversion of phenylalanine to tyrosine. In an attempt to break down the phenylalanine, other enzymes in the cells convert it to phenylpyruvate. If phenylalanine and phenylpyruvate accumulate in the blood of an infant, it can lead to severe brain damage and mental retardation. If PKU is detected in a newborn baby, a diet is prescribed that eliminates all the foods that contain phenylalanine. Preventing the buildup of the phenylpyruvate ensures normal growth and development. 

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. 

Nitisinone is a reversibile inhibitor of 4-hydroxy-phenylpyruvate oxidase, an enzyme that plays a crucial role in the tyrosine catabolic pathway. Nitisinone prevents the accumulation of the toxic metabolites fumaryl acetoacetate, succinyl acetoacetate and succinyl acetone. Nitisinone is used for the treatment of hereditary tyrosinemia type 1. After oral administration bioavailability is 90% and peak levels are reached at 2.5 hours after dosing. The drug is eliminated mainly in the urine but some CYP3A4-mediated metabolism seems to occur. The elimination half-life is 45 hours. Blood dyscrasias are frequently occurring side effects as are eye problems like conjunctivitis, corneal opacity and keratitis. Exfoliative dermatitis, erythematous rash and pruritus... 

Correct answer = B. Alkaptonuria is a rare metabolic disease involving a deficiency in homogentisic acid oxidase, and the subsequent accumulation of homogentisic acid in the urine, which turns dark upon standing. The elevation of methylmalonate (due to methylmalonyl CoA mutase deficiency), phenylpyruvate (due to phenylalanine hydroxlyase deficiency), a-ketoisovalerate (due to branched-chain a-ketoacid dehydrogenase deficiency), and homocystine (due to cystathionine synthase deficiency) are inconsistent with a healthy child with darkening of the urine. 

They lack the enzyme phenylalanine oxidase, which converts phenylalanine to tyrosine. Thus phenylalanine accumulates in the body and it is degraded to phenylpyruvate by transamination ... 

Additional errors of phenylalanine and tyrosine metabolism include tyrosinosis, or hereditary tyrosinemia, neonatal tyrosinemia, and alcaptonuria. In the first case, there is a probable defect in p-hydroxyphenylpyruvate oxidase. In neonatal tyrosinemia, the problem is transient and may be solved by the administration of ascorbic acid. Ascorbic acid is apparently a cofactor for p-hydroxy-phenylpyruvate oxidase. Alcaptonuria is a benign disorder in which homogen-tisic acid oxidase is inoperative and homogentisic acid is excreted in the urine. Air oxidizes the homogentisic acid to a pigment, giving urine a black color. This pigment also accumulates in the patient s tissues. 

As illustrated in Figure 19-1, there are two different competing pathways available for the catabolism of PA. One pathway involves transamination of PA to phenylpyruvate, followed by the phenylpyruvate dehydrogenase reaction to form phenylacetyl-CoA. The products of these reactions (phenylpyruvate, phenylacetyl-CoA) and the excretion products phenylacetate and phenyllactate cannot be further metabolized and consequently accumulate in the blood, urine, and tissues. 

The first enzyme activity (dihydrobiopterin reductase) catalyzes the transfer of hydrogen to dihydrobiopterin, which is thus reduced to tetrahydrobiopterin. The second enzyme activity is a hydroxylase containing two Fe3+ atoms, and this catalyzes the reduction of Oz such that one oxygen atom is incorporated into phenylalanine to form tyrosine and the second into water. At the same time tetrahydrobiopterin is oxidized to dihydrobiopterin. Phenylalanine hydroxylase is an example of a mixed-function oxidase. An inherited deficiency of phenylalanine hydroxylase results in the accumulation of phenylalanine that is not converted to tyrosine but is excreted as phenylpyruvate. This condition, which affects young infants, is known as phenylketonuria and is associated with severe mental retardation. 

Phenylketonuria is due to an inborn error of phenylalanine metabolism. Typically, it is due to a deficiency of phenylalanine hydroxylase. Atypically, it can be caused by a deficiency of dihydrobiopterin reductase and a resultant inability to synthesize biopterin. All these conditions cause an accumulation of phenylalanine, which can be transaminated to phenylpyruvic acid. 

Certain phenylalanine auxotrophs of E. coli were observed to possess the unusual ability of developing a growth factor for themselves. The accumulation of a nutritionally inactive compound, its spontaneous decomposition to phenylpyruvic acid (as the pH of the growth medium dropped), and the conversion of the latter to phenylalanine, were shown to be responsible for this effect. The unstable compound, prephenic acid, was isolated from culture filtrates and was characterized as XVAlthough its half-life at room temperature in N hydrochloric acid was 1 noinute, prephenic acid was relatively stable at neutral pH. Its enzymic conversion... 

Phenylketonuria is perhaps the best known of the diseases of amino acid metabolism. Phenylketonuria is caused by an absence or deficiency of phenylalanine hydroxylase or, more rarely, of its tetrahydrobiopterin cofactor. Phenylalanine accumulates in all body fluids because it cannot be converted into tyrosine. Normally, three-quarters of the phenylalanine is converted into tyrosine, and the other quarter becomes incorporated into proteins. Because the major outflow pathway is blocked in phenylketonuria, the blood level of phenylalanine is typically at least 20-fold as high as in normal people. Minor fates of phenylalanine in normal people, such as the formation of phenylpyruvate, become major fates in phenylketonurics. 

Deficiency of phenylalanine hydroxylase, tetrahydrobiopterin, or dihydropteridine reductase results in phenylketonuria (PKU), an autosomal recessive trait. Because phenylalanine accumulates in tissues and plasma (hyperphenylalaninemia), it is metabolized by alternative pathways and abnormal amounts of phenylpyruvate appear in urine (Figure 17-22). Phenylalanine hydroxylase deficiency may be complete (classic PKU, type 1) or partial... 

Phenylketonuria, caused by a deficiency of phenylalanine hydroxylase, is one of the most common genetic diseases associated with amino acid metabolism. If this condition is not identified and treated immediately after birth, mental retardation and other forms of irreversible brain damage occur. This damage results mostly from the accumulation of phenylalanine. (The actual mechanism of the damage is not understood.) When it is present in excess, phenylalanine undergoes transamination to form phenylpyruvate, which is also converted to phenyllactate and phenyl-acetate. Large amounts of these molecules are excreted in the urine. Phenylacetate gives the urine its characteristic musty odor. Phenylketonuria is treated with a low-phenylalanine diet. 

Tyrosine Oxidation (p-Hydroxyphenylpyruvate Oxidase) (Eq. 17) The interruption of tyrosine degradation at the stage of p-hydroxy-phenylpyruvate (pHPP) in scurvy has long been considered Ae clearest evidence of a biochemical role for ascorbic acid. The intmmediate accumulates and is excreted when doses of tyrosine are given to scorbutic man, other primates, and guinea pigs, and to premature infants (K8). 

The incidence ol phcnylkeionuiiu is around one in every ten thousand births in the UK. Phenylketonuria arises from impaired conversion of phenylalanine to tyrosine, usually because of a deficiency of phenylalanine hydroxylase. Figure 5 shows how phenylalanine, an essential amino acid, is metabolized. In phenylketonuria. phenylalanine cannot be converted to tyrosine, accumulates in blood and is excreted in the urine. The main urinary metabolite is phenylpyruvic acid (a phenylketone ) which gives the disease its name. The clinical features include ... 

Mutations leading to deficiencies in enzymes are usually referred to as inborn errors of metabolism, because they involve defects in the DNA of the affected individual. Errors in enzymes that catalyze reactions of amino acids frequendy have disastrous consequences, many of them leading to severe forms of mental retardation. Phenylketonuria (PKU) is a well-known example. In this condition, phenylalanine, phenylpyruvate, phenyllactate, and phenylacetate all accumulate in the blood and urine. Available evidence suggests that phenylpyruvate, which is a phenylketone, causes mental retardation by interfering with the conversion of pyruvate to acetyl-CoA (an important intermediate in many biochemical reactions) in the brain. It is also likely that the accumulation of these products in the brain cells results in an osmotic imbalance in which water flows into the brain cells. These cells expand in size until they crush each other in the developing brain. In either case, the brain is not able to develop normally. 

Reactions involved in the development of phenylketonuria (PKU). A deficiency in the enzyme that catalyzes the conversion of phenylalanine to tyrosine leads to the accumulation of phenylpyruvate, a phenyl ketone. 

A small number of infants who have phenylketonuria have normal levels of phenylalanine hydroxylase activity, but on normal diets they continue to accumulate phenylalanine as well as other metabolites, including phenylpyruvate, phenyllactate, and phenylacetate. They also have high levels of quinonoid dihydrobiopterin. 

At least 35 different hereditary changes in amino acid metabolism have been identified. One of the more well known is an inherited disease associated with abnormal aromatic amino acid metabolism. In phenylketonuria (PKU), there is a lack of the enzyme phenylalanine hydroxylase. As a result, phenylalanine cannot be converted to tyrosine, leading to the accumulation of phenylalaifine and its metabolites (phenylpyruvate and phenyl-acetate) in the tissues and blood ... 

Perhaps the most successful and widely used artificial sweetener is aspartame, the methyl ester of a dipeptide formed from phenylalanine and aspartic acid. Aspartame is roughly 100 times as sweet as sucrose. It undergoes slow hydrolysis in solution, however, which limits its shelf life in products such as soft drinks. It also cannot be used for baking because it decomposes with heat. Furthermore, people with a genetic condition known as phenylketonuria cannot use aspartame because their metabolism causes a buildup of phenylpyruvic acid derived from aspartame. Accumulation of phenylpyruvic acid is harmful, especially to infants. Alitame, on the other hand, is a compound related to aspartame, but with improved properties. It is more... 

PKU is a genetic disorder characterised by deficient metabolism of phenylalanine, resulting in the accumulation of phenylalanine and the ketone, phenylpyruvate. Neonatal screening (recently improved by the introduction of tandem mass spectrometry) for PKU assists diagnosis and treatment, which reduces the risk of mental retardation associated with this disorder. 

The accumulating metabolite is metabolized through alternative pathways, mainly the transaminase reaction. Consequently, imidazole pyruvic acid accumulates in the urine. The imidazole pyruvic, like phenylpyruvic, acid reacts with ferric chloride to yield a blue compound. As a result, the diaper test does not distinguish between phenylketonuria and histidinemia. Yet the diagnosis is of considerable importance because histidinemia is a much more benign disease. Furthermore, histidinemia is not alleviated by withdrawal of phenylalanine from the diet. 

In the hereditary disease phenylpyruvic oligophrenia there appears to be an inability to convert phenylalanine to tyrosine, which results in an accumulation of phenylalanine in the blood, and the excretion of phenylalanine, phenylpyruvic acid, and phenyllactic acid. ... 

Some inherited metabolic disorders are extremely damaging while the effects of others are so mild that they can hardly be called diseases. For example, alkaptonurics are usually quite healthy although in later life they are prone to a particular form of arthritis. Similarly the conditions of fructosuria and pentosuria, in which fructose and pentose sugars respectively appear in the urine, have no pathological consequences. At the other end of the scale is phenylketonuria in which the enzyme phenylalanine hydroxylase, which is responsible for converting phenylalanine to tyrosine, is lacking. In this condition phenylpyruvic acid and other intermediate products of phenylalanine metabolism accumulate in the blood and tissues and are so injurious to the central nervous system that, although physical development is essentially normal, there is severe mental retardation. 

An inborn error of metabolism in which there is a deficiency of phenylalanine hydroxylase, the enzyme which converts phenylalanine to tyrosine. As a result, phenylalanine accumulates in the body and is converted to a number of derivatives such as phenylpyruvic acid, phenyllactic acid and phenylacetic acid which are excreted in the urine. If the condition is untreated, mental retardation may result. It has an autosomal recessive mode of inheritance. 

In Escherichia coli. Salmonella typhimurium and Aerobacter aerogenes two soluble multi-activity enzymes or enzyme complexes function in the utilisation of chorismate (14) for L-phenyl-alanine and L-tyrosine synthesis An enzyme or enzyme complex (P-protein) containing chorismate mutase and prephenate dehydratase activities has been isolated and partially purified from Escherichia coli. Salmonella typhimurium and Aerobacter aerogenes. The enzyme complex catalyses the transformation of chorismate (14) to phenylpyruvate (32) and both enzymic activities are retained in physical association after chromatography on DEAE cellulose. Kinetic analysis indicated that in isolated enzyme systems direct synthesis of phenylpyruvate (32) from chorismate (14) does not occur. Prephenate (31) once formed dissociates from the enzyme surface and accumulates in the reaction medium. After a lag period it is converted to phenylpyruvate (32). Schmit, Artz and Zalkin also obtained evidence to show that functionally distinct sites (catalytic and regulatory) exist on the P-protein from Salmonella typhimurium for chorismate mutase and prephenate dehydratase activities. The P-protein was obtained from Escherichia coli K-12 by Davidson, Blackburn and Dopheide who showed that it existed in solution mainly as a dimer of similar (and probably identical) sub-units of... 

Several inborn errors of metabolism are concerned with the metabolism of L-phenylalanine and L-tyrosine in mammals. In several cases it has been possible to demonstrate that such biochemical disorders are associated with the absence or partial deficiency of a particular enzymatic activity. Phenylketonuria results from the absence of a normal L-phenylalanine hydroxylase activity and individuals suffering from this disease are unable to convert L-phenylalanine to L-tyrosine. Under these conditions the metabolism of the amino acid to phenylpyruvic acid, phenyl-lactic acid and phenyl acetyl glutamine is greatly exaggerated. Phenylketonuria is a severe disorder and results in a marked mental retardation, particularly in children. It is generally assumed that it is the accumulation of abnormal metabolites which is responsible for the mental symptoms associated with the disease. 

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