Phenylpyruvate excretion

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. 

These conditions can be treated with a low protein diet and administration of sodium benzoate or phenylpyruvate to provide an alternative route for capturing and excreting excess nitrogen. 

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 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

DISCUSSION. Caution should be exercised when interpreting chromatograms from a complex mixture of aromatic compounds in urine. This is especially true when excretion of the catechols is very high. When the derivatives of the compound extracted from urine of patients with phenylketonuria are prepared decomposition products of the large amounts of phenylpyruvic acid present give rise to peaks which interfere with and resemble HVA. When difficulties of this type are suspected, differential extraction of the urine can be performed. When alkaptonuria urine is extracted with dichloromethane, homovanillic acid is extracted preferentially, leaving the spurious compounds behind. 

An inability to degrade amino acids causes many genetic diseases in humans. These diseases include phenylketonuria (PKU), which results from an inability to convert phenylalanine to tyrosine. The phenylalanine is instead transaminated to phenylpyruvic acid, which is excreted in the urine, although not fast enough to prevent harm. PKU was formerly a major cause of severe mental retardation. Now, however, public health laboratories screen the urine of every newborn child in the United States for the presence of phenylpyru-vate, and place children with the genetic disease on a synthetic low-phenylalanine diet to prevent neurological damage. 

Phenylpyruvate is excreted in the urine. Normal urine does not contain any phenylpyruvate People suffering from PKU have varying amounts of phenylpyruvate in their urine. PKU causes severe mental retardation in infants if it is not treated immediately after birth, which is done by restricting the phenylalanine content of the diet. In many states, the law requires that every newborn be tested for phenylpyruvate in the urine. The test is based on the reaction of the iron (III) ion with the phenylpyruvate, producing a gray-green color. 

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. 

The disease phenylketonuria, which causes severe mental retardation, is characterized by the urinary excretion of phenylpyruvate. Why is this formed ... 

Phenylpyruvate can be reduced to phenyl lactate and oxidatively decarboxylated to phenylacetate. both of which are also excreted in the urine. 

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,... 

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). 

Figure 55-8 Partial urine organic acid profiles 15-23 minute portion of a 33 minute run) of two patients with tyrosinemia type i. A, Acutely III patient with markedly elevated excretion of succiny[acetone, pre-NTBC treatment.The insert shows the selected ion chromatogram of the [M-15] ion of succinylacetone O-TMS-oxime TMS ester, m/z 212 B, Fifteen month old patient, succinylacetone was not detected by either total ion current (orrow) or selected ion chromatogram in three different urine specimens.This patient was later shown to be compound heterozygote for the French Canadian common splice mutation (IVS12+5G>A) and another previously unreported mutation. Peak legend I, Succinylacetone (oxime, peak I) 2, succinylacetone (oxime, peak II) 3, 4-hydroxy phenyllactic acid 4, 4-hydroxy phenylpyruvic add (oxime).The symbol marks the internal standard (pentadecanoic acid), signal abundance is normalized to the intensity of the internal standard peak.

The urine also contains considerable amounts of phenylalanine (180, 274), phenyllactic acid (150, 151), and phenylacetylglutamine (832, 960), typical urinary excretion values being (960), in milligrams per 100 ml., phenylalanine 38, phenylpyruvic acid 50, phenyllactic acid 54, and phenylacetylglutamine 53. Phenylacetylglutamine occurs in normal urine in appreciable quantities (0.25 to 0.5 g. per day), but excretion by the phenylketonuric is much higher (e.g., 2.4 g. per day, ref. 832). 

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. 

Besides an increase in urinary phenylalanine, phenylketonuric patients have a large excretion of phenylpyruvic and phenyllactic acids (Jl), as well as of phenylacetic acid conjugated as phenylacetylglutamine (W16). 

The increase in the urinary excretion of phenylpyruvic acid has been currently used as a means of detecting phenylketonuria, and a great many techniques have been described for the determination of this rather unstable acid (B14, K8, K9, K16, M19, M20, RIO, S2, T9). Phenylpyruvic acid has also been detected in the blood of such patients... 

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). 

Phenylalanine, an essential amino acid, can be converted to tyrosine in the liver, catalyzed by the enzyme phenylalanine hydroxylase. If phenylalanine hydroxylase is genetically insufficient, phenylketonuria results, with the excretion of phenylpyruvate. In this disease, tyrosine becomes an essential amino acid. This disease can be treated by feeding low-phenylanine diets. 

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 ... 

It all works much better if we consider some simpler cases that are less obvious in ordinary life but still have greater importance for medicine than skin or eye color. Take, for example, the disease phenylketonuria, which 1 mentioned briefly in Chapter 8. If left untreated it produces severe mental retardation and, in many cases, death before the age of 25. It is caused by an incapacity to convert the aminoacid phenylalanine into another aminoacid, tyrosine. It is not, however, a deficiency disease, because its harmful effects are not caused by a shortage of tyrosine, and cannot be avoided by adding tyrosine to the diet. Instead they are caused by the toxic effects on the brain of a substance called phenylpyruvate, which the body produces in its efforts to remove the excess of phenylalanine. The name of the disease reflects the fact that phenylpyravate, which belongs to a general class of chemical substances known as phenylketones, is excreted in the urine of affected people. This provides a simple method of diagnosis, and the disease is treated by carefully controlling the diet so that it provides no more phenylalanine than is needed for normal health. There is then no surplus to be converted into phenylpyruvate. 

Phenylalanine can also be transaminated to phenylpyruvic add as an alternative to hydroxylation by phenylalanine hydroxylase. Phenylpyruvic acid, along with other ketones, is excreted in the urine as phenylacetic acid, phe-acetylglutamine and phenyllactic acid. This pathway of phenylalanine metabolism is much less effective than hydroxylation [1,4]. 

Classical phenylketonuria is an hereditary defect in the synthesis of Phe hydroxylase (the enzyme may be absent or inactive), which affects about 1 infant in 10,000. These individuals are unable to convert Phe into tyrosine, and the major route of Phe metabolism is thus blocked. Phenylpyruvate and phenylacetic acid are excreted in the urine. The condition is accompanied by defective pigmentation and, if untreated, by severe mental retardation (hence the other name, phenylpyruvic oligophrenia, also known as Polling s syndrome). Tlie urine of newborn infants is now routinely tested (Guthrie test) for the presence of phenylketones the condition can be compensated by a diet low in phenylalanine, and the typic mental retardation is thereby avoided. Other types of phenylketonuria are due to defective reduction or synthesis of dihydrobiopterin (see Inborn errors of metabolism). 

Deoxynorlaudanosoline carboxylic acid is formed (in a spontaneous reaction ) from DOPA and phenylpyruvic acid in children with phenylketonuria. It is excreted with the urine (E 1). 

Most patients continuously excreting phenylpyruvic acid and other related metabolites in the urine are feeble minded usually they fall into the class of idiots or imbeciles. However, they show no paralysis or increase in muscular tone, despite markedly accentuated superficial and deep reflexes. 

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