Hydroxyphenylpyruvate hydroxylase

The probable metabohc defect in type I tyrosine-mia (tyrosinosis) is at himarylacetoacetate hydrolase (reaction 4, Figure 30-12). Therapy employs a diet low in tyrosine and phenylalanine. Untreated acute and chronic tyrosinosis leads to death from liver failure. Alternate metabolites of tyrosine are also excreted in type II tyrosinemia (Richner-Hanhart syndrome), a defect in tyrosine aminotransferase (reaction 1, Figure 30-12), and in neonatal tyrosinemia, due to lowered y>-hydroxyphenylpyruvate hydroxylase activity (reaction 2, Figure 30-12). Therapy employs a diet low in protein. 

The metabolism of phenylalanine will now be considered in some detail, as two inborn errors of metabolism are known that affect this pathway. Phenylalanine is first hydroxylated by phenylalanine hydroxylase to form another aromatic amino acid tyrosine (Fig. 8). The coenzyme for this reaction is the reductant tetrahydrobiopterin which is oxidized to dihydrobiopterin. Phenylalanine hydroxylase is classified as a monooxygenase as one of the atoms of 02 appears in the product and the other in HzO. The tyrosine is then trans-aminated to p-hydroxyphenylpyruvate, which is in turn converted into homogentisate by p-hydroxyphenylpyruvate hydroxylase. This hydroxylase is an example of a dioxygenase, as both atoms of 02 become incorporated into the product (Fig. 8). The homogentisate is then cleaved by homogentisate oxidase, another dioxygenase, before fumarate and acetoacetate are produced... 

The next step in the degradation of phenylalanine and tyrosine is the transamination of tyrosine to p-hydroxyphenylpyruvate (Figure 23.30). This a-ketoacid then reacts with Oj to form homogentisate. The enzyme catalyzing this complex reaction, p-hydroxyphenylpyruvate hydroxylase, is called a... 

Saito, I., Y. Chujo, H. Shimazu, M. Yamane, T. Matsuura, and H. J. Cahnmann Non Enzymic Oxidation of p-Hydroxyphenylpyruvic Acid with Singlet Oxygen to Homogentisic Acid. A Model for the Action of p-Hydroxyphenylpyruvate Hydroxylase. J. Amer. Chem. Soc. 97, 5272 (1975). 

Goswami, M.N.D., Rosenberg, A.J. and Meury, F. (1973), A comparative analysis of the ontogenic development of rat liver sequential enzymes - Tyrosine a-ketoglutarate aminotransferase, p-hydroxyphenylpyruvate hydroxylase, and homogentisate oxygenase. Dev. Biol., 30,129. 

The liver is also the principal metabolic center for hydrophobic amino acids, and hence changes in plasma concentrations or metabolism of these molecules is a good measure of the functional capacity of the liver. Two of the commonly used aromatic amino acids are phenylalanine and tyrosine, which are primarily metabolized by cytosolic enzymes in the liver [1,114-117]. Hydroxylation of phenylalanine to tyrosine by phenylalanine hydroxylase is very efficient by the liver first pass effect. In normal functioning liver, conversion of tyrosine to 4-hy-droxyphenylpyruvate by tyrosine transaminase and subsequent biotransformation to homogentisic acidby 4-hydroxyphenylpyruvic acid dioxygenase liberates CO2 from the C-1 position of the parent amino acid (Fig. 5) [1,118]. Thus, the C-1 position of phenylalanine or tyrosine is typically labeled with and the expired C02 is proportional to the metabolic activity of liver cytosolic enzymes, which corresponds to functional hepatic reserve. Oral or intravenous administration of the amino acids is possible [115]. This method is amenable to the continuous hepatic function measurement approach by monitoring changes in the spectral properties of tyrosine pre- and post-administration of the marker. 

Figure 10 Several reactions catalyzed by a-keto acid-dependent iron enzymes, (a) prolyl 4-hydroxylase, (b) deacetoxycephalosporin C synthase (DAOCS), (c) 4-hydroxyphenylpyruvate (HPP) dioxygenase and (d) clavaminate synthase 2 (CAS)...

Ascorbate increases the activity of hydroxylases needed for the conversion of p-hydroxyphenylpyruvate to homogentisate (Chapter 17), synthesis of norepinephrine from dopamine (Chapter 32), and two reactions in carnitine synthesis (Chapter 18). It is not known whether decreased activity of these enzymes contributes to the clinical characteristics of scurvy. Although ascorbic acid is needed for maximal activity of these enzymes in vivo and in vitro, most show some activity when other reducing agents are used. 

Fates of tyrosine. Tyrosine can be degraded by oxidative processes to ace-toacetate and fumarate which enter the energy generating pathways of the citric acid cycle to produce ATP as indicated in Figure 38-2. Tyrosine can be further metabolized to produce various neurotransmitters such as dopamine, epinephrine, and norepinephrine. Hydroxylation of tyrosine by tyrosine hydroxylase produces dihydroxyphenylalanine (DORA). This enzyme, like phenylalanine hydroxylase, requires molecular oxygen and telrahydrobiopterin. As is the case for phenylalanine hydroxylase, the tyrosine hydroxylase reaction is sensitive to perturbations in dihydropteridine reductase or the biopterin synthesis pathway, anyone of which could lead to interruption of tyrosine hydroxylation, an increase in tyrosine levels, and an increase in transamination of tyrosine to form its cognate a-keto acid, para-hydroxyphenylpyruvate, which also would appear in urine as a contributor to phenylketonuria. 

E. The correct response is very low levels of phoiylalanine hydroxylase, a key oizyme in the metabolic sequelae of phenylketonuria, that is, elevated phenylalanine, phoiylpyruvale, and para-hydroxyphenylpyruvate in blood. Homogentisic acid is an intermediate in the breakdown of tyrosine to fumarate and acetoacetate. Vitamin is required in the metabolism of branched-chain amino acids not phenylalanine. The a-keto acids of the branched chain amino acids produce the maple-syrup odor. 

Phenylalanine hydroxylase is like p-hydroxyphenylpyruvate oxidase (see below) in its requirement for two enzyme components. It is particularly interesting that one of these components (Fraction II) can be replaced by acetone powder extracts of liver from rabbit, calf, dog, and pig, and by acetone powder extracts of kidney and heart from calf and hog, although these oi ans contain no phenylalanine hydroxylase activity. Fraction II appears to be wide distributed, and to have fimctions other than these connected with phenylalanine hydroxylation. Although two enzyme components are involved in the system, no evidence for an intermediate between phenylalanine and tyrosine has been found (557), nor has it been possible to separate the process into two steps. 

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