Phenylketonuria, enzymic defect

The intact animal can be improved for experimental purposes if it is rendered abnormal in some way, by genetic malfunction, by illness, or by operation. Genetic defects, or mutations, are used widely in the study of bacterial metabolism, where they can be read ily induced, for example through irradiation by X-rays or from a radioactive source. Genetic defects frequently reveal themselves in the form of the absence of one specific enzyme, and metabolic studies with such enzymically defective preparations are of the same type as those made possible by the use of a specific enzymic inhibitor which we discussed above. Genetic defects in animals are rarer, but classic cases of the absence of specific enzymes and hence the accumulation of abnormal metabolites are provided in humans by the genetically carried diseases of phenylketonuria and alkaptonuria. In both, unusual substances are excreted in the urine, and the analysis of the reasons for their appearance has led to valuable information about the mechanism of amino acid metabolism in the body. 

Artificial cells have been used in hereditary enzyme defects, including our earliest use of a replacement for catalase in acatalasemic mice. This also has been studied for asparagine removal in the treatment of leukemia in animals."" We used phenylalanine ammonia lyase artificial cells in phenylketonuria rats." Later, we found an extensive enterore-circulation of amino acids in the intestine." This allows enzyme artificial cells to be used orally to selectively remove specific amino acids from the body, as in phenylketonuria." We also studied the oral administration of artificial cells containing xanthine oxidase." " This resulted in a decrease in systemic hypoxanthine in a pediatric patient with hypoxanthinuria (Lesch-Nyhan disease). 

In cases of phenylketomnia, an enzyme defect leads to large increases of phenylalanine in blood and tissue, with consequent damage to the brain. The only possible therapy is a diet of low phenylalanine content. Phenistix , used for diagnosis of phenylketonuria, are too inaccurate for adequate control moreover it is a matter of normalising not only the phenylalanine but the whole amino acid pattern in the serum and elimination in urine. Only column chromatography has so far been capable of fulfilling these requirements. The introduction of TLC could effect a real simplification. 

The enzymic defect in phenylketonuria has been traced to a lack of component I of the hydroxylating enzyme system 199, 19S). 

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. 

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). 

Phenylketonuria can also be caused by a defect in the enzyme that catalyzes the regeneration of tetrahydrobiopterin (Fig. 18-24). The treatment in this case is... 

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.

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,... 

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. 

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). 

The metabolic defect involved in alkaptonuria w as suggested by Bateson (34) as early as 1902 to be inherited as a recessive Mendelian character, and later evidence has supported this prediction (395, 643). Gross (322) in 1914 concluded that it was due to lack of a specific enzyme. Alkaptonuria, unlike phenylketonuria, is not accompanied by mental symptoms and is not an incapacitating disorder except insofar as it may lead to ochronosis and arthritis (c/. 598). 

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. 

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 ... 

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. 

Phenylketonuria (PKU) is an inborn error in phenylalanine metabolism caused by a deficiency of the phenylalanine hydroxylase (PAH) enzyme (Fig. 10.1). The cofactor for PAH is tetrahydrobi-opterin (BH4). In PKU, blood concentrations of phenylalanine accumulate, affecting myelin and neurotransmitter production (Box 10.1) [1,2], With the defect in PAH, phenylalatfine is not converted to tyrosine thus, tyrosine becomes a conditionally essential amino acid and must be... 

Orthomolecular psychiatric therapy is the treatment of mental disease by the provision of the optimum molecular environment for the mind, especially the optimum concentrations of substances normally present in the human body ). An example is the treatment of phenyl-ketonuric children by use of a diet containing a smaller than normal amount of the amino acid phenylalanine. Phenylketonuria (2) results from a genetic defect that leads to a decreased amount or effectiveness of the enzyme catalyzing the oxidation of phenylalanine to tyrosine. The patients on a normal diet have in their tissues abnormally high concentrations of phenylalanine and some of its reaction products, which, possibly in conjunction with the decreased concentration of tyrosine, cause the mental and physical manifestations of the disease (mental deficiency, severe eczema, and others). A decrease in the amount of phenylalanine ingested results in an approximation to the normal or optimum concentrations and to the... 

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). 

INBORN ERRORS OF THE METABOLISM. At times the metabolism of the nutrients cannot proceed normally due to some defect in the genetic information that exists at birth or shortly thereafter. These defects can affect the metabolism of carbohydrates, proteins, and fats hence, they are referred to as inborn errors of metabolism. Often they are due to production of a nonfunctional enzyme or complete lack of an enzyme involved in the metabolic scheme. Since enzymes are protein, their production relies upon correct genetic information. Many of these inborn errors have serious consequences, but fortunately most are rare. Familiar examples of errors in carbohydrate metabolism include lactose intolerance and galactosemia. Familiar examples of errors in protein metabolism include albinism, maple syrup urine disease, and phenylketonuria. The hyperlipoproteinemias are familiar examples of inborn errors of fat metabolism. 

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. 

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