Homogentisate oxidation

This compound differs slightly from the isomeric fumarylacetoacetate in its absorption spectrum near 300 mu at neutral and alkaline pH values, but the maleyl compound has essentially no absorption at pH 1 whereas the fumaryl compound has a strong peak at 310 miu in acid. Fumarylacetoacetate had previously been isolated as a product of homogentisic oxidation by Ravdin and Crandall. Knox and collaborators have shown that the fumaryl compound is formed secondarily by a cis-trans isomerase, an enzyme that requires glutathione for activity and has in addition essential sulfhydryl groups on the protein. The isomerization has not been reversed. 

C. Excreted in the urine in the rare hereditary disease alkaptonuria. Homogentisic acid is easily oxidized in the air to dark-coloured polymeric products, so that urine from patients with alkaptonuria turns gradually black. It is formed from tyrosine and is an intermediate in tyrosine breakdown in the body. Alkaptonuria is due to the absence of the liver enzyme which cleaves the aromatic ring. 

Tyrosine. Figure 30-12 diagrams the conversion of tyrosine to amphibolic intermediates. Since ascorbate is the reductant for conversion of y>-hydroxyphenylpyru-vate to homogentisate, scorbutic patients excrete incompletely oxidized products of tyrosine catabohsm. Subsequent catabohsm forms maleylacetoacetate, fu-marylacetoacetate, fumarate, acetoacetate, and ultimately acetyl-CoA>... 

Another inheritable disease of phenylalanine catabolism is alkaptonuria, in which the defective enzyme is homogentisate dioxygenase (Fig. 18-23). Less serious than PKU, this condition produces few ill effects, although large amounts of homogentisate are excreted and its oxidation turns the urine black. Individuals with alkaptonuria are also prone to develop a form of arthri-... 

The 2-oxoacid p-hydroxyphenylpyruvate is decar-boxylated by the action of a dioxygenase (Eq. 18-49). The product homogentisate is acted on by a second dioxygenase, as indicated in Fig. 25-5, with eventual conversion to fumarate and acetoacetate. A rare metabolic defect in formation of homogentisate leads to tyrosinemia and excretion of hawkinsin97 a compound postulated to arise from an epoxide (arene oxide) intermediate (see Eq. 18-47) which is detoxified by a glutathione transferase (Box 11-B). 

The fundamental role of ascorbic acid in metabolic processes is not well understood. There is some evidence that it may be involved in metabolic hydroxylation reactions of tyrosine, proline, and some steroid hormones, and in the cleavage-oxidation of homogentisic acid. Its function in these metabolic processes appears to be related to the ability of vitamin C to act as a reducing agent. 

The allyl group in the allylphenols can be oxidized, after protecting the hydroxyl group, to yield substituted phenylacetaldehydes 78 - 76 76 and phenylacetic acids. Thus, homogentisic acid LXIII is prepared readily by ozonizing the dibenzoate of allylhydroqumone LXIV, which is obtained by rearrangement of the allyl ether of hydroquinone mono-... 

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... 

In oxidation reactions of ascorbic acid, homogentisic acid and hydroquinone by poly(l-histidine) — Cu(II) complex, the reaction profile shows a Michaelis-Menten type curve in the reaction condition of pH ranging from 5—6. These oxidation reaction rates are higher than the oxidation rates by the catalyst without poly(l-histidine). On the other hand, the rate of oxidation of a positively charged substrate, phenylenedi-... 

Table 18. Oxidation of ascorbic acid, homogentisic acid and p-hydroquinone by poly(L-Iysine)-Cu(II) complex...

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. 

Early experiments by Bernheim, Felix, Sealock, and their co-workers on oxidation of tyrosine by liver breis showed an uptake of four atoms of oxygen per mole of tyrosine, with the production of one molecule each of carbon dioxide and acetoacetate, but no ammonia (60, 61, 261, 262, 789, 976). Felix and Zorn (261) found alanine to be formed and considered this to arise from a direct splitting of the tyrosine side chain. Although the experiments with man and intact animals already described made it seem very probable that p-hydroxyphenylpyruvic acid and homogentisic acid were normal intermediates in tyrosine metabolism, and although homogentisic acid was known to be readily metabolized by normal liver (e.g., 208, 695, 976) Felix and co-workers (262) considered p-hydroxyphenylpy-ruvic acid and homogentisic acid not to be intermediates in the breakdown of tyrosine by the liver system. 

Alcaptonuria is a rare metabolic hereditary disease in which homogentisic acid is eliminated in urine, which darkens upon exposure to air owing to oxidation of... 

The list of acids found by Armstrong et al. (A14) in urine is not exhaustive. One of the solvents used for chromatography leads, indeed, to the decomposition of many easily oxidized dihydroxyphenyl compounds, so that other techniques are necessary to characterize these substances, among which would be homogentisic, homoprotocatechuic, and caffeic acids the same holds true for o- and p-hydroxy- as well as... 

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... 

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