4- Hydroxyphenylpyruvate activity

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 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. 

The molecular target site of triketone herbicides is the enzyme -hydroxyphenylpyruvate dioxygenase (HPPD). Inhibition of this enzyme disrupts the biosynthesis of carotenoids and causes a bleaching (loss of chlorophyll) effect on the foliage similar to that observed with inhibitors ofphytoene desaturase (e.g. norflurazon). However, the mechanism of action of HPPD inhibitors is different. Inhibtion of HPPD stops the synthesis of homogen tisate (HGA), which is a key precursor of the 8 different tocochromanols (tocopherols and tocotrienols) and prenyl quinones. In the absence of prenylquinone plastoquinone, phytoene desaturase activity is interrupted. The bleaching of the green tissues ensues as if these compounds inhibited phytoene desaturase. 

Meazza G, Scheffler BE, Tellez MR, Rimando AM, Nanayakkara NPD, Khan LA, Abourashed EA, Romagni JG, Duke SO, Dayan EE, The inhibitory activity of natural products on plant -hydroxyphenylpyruvate dioxygenase. Phytochemistry 59 281-288, 2002. 

Prephenate dehydrogenase [EC 1.3.1.12] catalyzes the reaction of prephenate with NAD+ to produce 4-hydro-xyphenylpyruvate, carbon dioxide, and NADH. This enzyme in enteric bacteria also possesses chorismate mutase activity and converts chorismate into prephenate. Prephenate dehydrogenase (NADP+) [EC 1.3.1.13] catalyzes the reaction of prephenate with NADP+ to produce 4-hydroxyphenylpyruvate, carbon dioxide, and NADPH. 

Operation of the NIH shift can cause migration of a large substituent as is illustrated by the hydroxylation of 4-hydroxyphenylpyruvate (Eq. 18-49), a key step in the catabolism of tyrosine (Chapter 25). Human 4-hydroxyphenylpyruvate dioxygenase is a dimer of 43-kDa subunits.439 A similar enzyme from Pseudomonas is a 150-kDa tetrameric iron-tyrosinate protein, which must be maintained in the reduced Fe(II) state for catalytic activity.440 Although this enzyme is a... 

Determination of p-hydroxyphenylpyruvate dioxygenase (HPPD) activity in vitro... 

Figure 3. Effect of the p-triketone (-)-usnic acid (circles), the benzoquinone sorgoleone (triangles) and the commercial herbicide sulcotrione (squares) on the activity of p-hydroxyphenylpyruvate dioxygenase. The dotted line represents 50% inhibition of enzyme activity.

Uchida, Suzuki, and Ichihara (878) isolated a soluble enzyme system (thereby possibly excluding mitochondrial participation) from rabbit liver, and partially purified it. Two enzymes were involved. The first of these converted p-hydroxyphenylpyruvic acid to 2,5-dihydroxyphenylpyruvic acid. If this enzyme was resolved, vitamin C alone did not restore the activity, but vitamin C and vitamin B12 did. The amount of B12 required was very low, and they suggested that the true enzyme was a Bw derivative, possibly aquocobalamin hydroxide bound to enzyme protein, and that the function of the ascorbic acid was solely to stabilize the reactive form of the coenzyme. This agrees with the work of La Du and Greenberg (524), who considered the role of ascorbic acid to be quite unspecific. Ascorbate increased the rate of tyrosine oxidation in liver preparations but the net consumption was zero, and moreover numerous ene-diols were just as effective on a molar basis. La Du and Greenberg considered that ascorbic acid participates in a cyclic oxidation-reduction and happens to be a substance of the correct oxidation-reduction potential either to participate directly or to protect some other participant. 

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. 

Fig. 5. Changes of liver enzyme activities after tyrosine dosage of guinea pigs tyrosine-a-ketoglutarate transaminase (- -o- -), p-hydroxyphenylpyruvate oxidase (— x—). Each point is the average of homogenates from two animals. From Knox and Goswami (Kll).

The amino acids phenylalanine and its hydroxylated derivative, tyrosine, are both catabolised in the livers of animals to fumaric acid and acetoacetic acid via homogentisic acid. This is formed by the oxidation of 4-hydroxyphenylpyruvate, catalysed by the copper containing enzyme 4-hydroxyphenylpyruvate dioxygenase, which requires vitamin C for its activity. The complete sequence is shown in Figure 5.13. The dioxygenase is so called because both the atoms of the... 

As added inactive tyrosine and p-hydroxyphenylpyruvic acid caused a greater dilution of activity in the protein than in the alkaloid being synthesized from [f/- C]fructose a separation of these two activities in L. williamsii into compartments was suggested. 

As discussed in detail in Chapter 4.2, triketones exert their herbicidal mode of action by inhibition of 4-hydroxyphenylpyruvate dioxygenase (HPPD) [6]. Triketones are not the only herbicide class that have this mode of action, and it has retrospectively been shown that apparently structurally non-related heterocyclic commercial herbicides such as isoxaflutole (7, BALANCE and MERLIN ), and the rice herbicides pyrazolate (8, SANBIRD ) and benzobicyclon (9, SHOW-ACE ) also cause these bleaching symptoms by the same mode of action. However, a common feature of these herbicides, after metabolic activation to the active metabolites (7 ) [7], (8 ) [8] and (9 ) [9] is the presence of an acidic 1,3-dicarbonyl moiety, which is also present in triketones (Fig. 4.3.2). Triketones and related her-... 

The next metabolic step involves the transformation of P-hydroxyphenylpyruvate into homogentisic acid. This reaction is accomplished in two steps (1) side chain migration and the addition of the hydroxyl group to the ring and (2) oxidative decarboxylation of the side chain. Ascorbic acid or ascorbic acid derivatives activate the system in an unknown manner. A study of the enzymes involved in parahydroxyphenylpyruvic oxidation demonstrated that two protein fractions, neither of which is active alone, are needed. One of the protein fractions was identified as catalase however, the formation of hydrogen peroxide during the reaction was not established. 

The first step in tyrosine oxidation is a transamination to form p-hydroxyphenylpyruvic acid. Several groups of investigators independently showed a dependence of tyrosine oxidation on the presence of a keto acid. Knox and LeMay-Knox showed that a-ketoglutarate is a specific partner in the transamination and that pyridoxal phosphate is a cofactor in this reaction. Partial resolution of the transaminase allowed a demonstration of parallel restoration of transaminase activity and over-all tyrosine oxidation by addition of pyridoxal phosphate. 

Purification of p-hydroxyphenylpyruvic oxidase led to the separation of two proteins, neither of which is active alone. One of these was identified as catalase. Catalase can be replaced as a component of the oxidase system by a peroxidase. These findings implicate H2O2 in the oxidation, and this is supported by the elimination of a lag period in the reaction by the addition of small quantities of H2O2. In spite of the requirement for two proteins and the effect of H2O2, an intermediate-level oxidant, no... 

In the metabolism of tyrosine, a deficiency of vitamin C will result in the build-up and excretion of the intermediary product, P-hydroxyphenylpyruvate, as a result of inactivating the enzyme P-hydroxyphenylpyruvic acid oxidase. When large amounts of tyrosine are being metabolized, vitamin C protects the enzyme P-hydroxyphenylpyruvic acid oxidase from inactivation (rather than activates as was formerly thought), and enhances the synthesis of norepinephrine, a neurotransmitter, from tyrosine. 

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