Maleylacetoacetic acid

Homogentisate oxidase catalyzes an important reaction In tyrosine metabolism, which converts the substrate homogentisic acid to the product maleylacetoacetic acid. 

Five enzyme steps have been demonstrated to be required in the conversion of tyrosine to fumaric and acetoacetic acid. These consist of a transamination to p-hydroxyphenylpyruvate, a simultaneous oxidation, migration of the side chain and decarboxylation to form homogentisic acid, oxidation of the latter to maleylacetoacetic acid, isomerization of this compound to fumarylacetoacetic acid, and hydrolysis of this acid to fumaric acid and acetoacetic acid. 

The next enzymic step in the dissimilation of tyrosine, oxidation of homogentisic acid, was clearly established by the work of Ravdin and Crandall 2H0). These investigators isolated two enzyme fractions from a rat liver homogenate, one of which catalyzed the oxidation of homogentisic acid to an open chain diketone-dicarboxylic acid. In their hands the product isolated was 4-fumarylacetoacetic acid. Subsequent work has shown that the initial product is 4-maleylacetoacetic acid and that an isomerase is present which converts this to the fumarylacetoacetate 221). The second enzyme of Ravdin and Crandall 220), fumarylacetoacetic acid hydrolase, hydrolytically cleaves this compound to fumarate and aceto-acetate. 

The nature of the reaction catalyzed by homogentisic oxidase was, not established immediately. Ravdin and Crandall (1951) isolated and identified fumarylacetoacetic acid as a product, but Knox and Edwards (1955b) subsequently found that the primary product is an isomer, maleylacetoacetic acid, that is converted enzymically to the compound of Ravdin and Crandall (Fig. 13). The formation of maleylacetoacetate... 

Fig. 11. The transformation of homogentisic acid into4-maleylacetoacetic acid.

Homogentisic acid is transformed into 4-maleylacetoacetic acid. Lerner (1953) supposed that the substrate is first oxidized to the corresponding quinone. Suda and Tokuyama (1958) have thorougly studied the influence of cations. The oxidase is a typical Fe++ enzyme. The inhibition with p-chloromercuribenzoate develops a complete iron requirement. This suggests that Fe is linked to an -SH group in accordance with the known ligand affinity (Crandall, 1955). HjOj is not likely to arise during oxidation. 

The mercapturic acids and related compounds can then be exported from cells by an ATP-dependent export pump. Glutathione is a coenzyme for glyoxalase (Eq. 13-33), maleylacetoacetate isomerase (Eq. 13-20), and DDT dehydrochlorinase. The latter enzyme catalyzes elimination of HC1 from molecules of the insecticide and is especially active in DDT-resistant flies.3 Glutathione is said to be the specific factor eliciting the feeding reaction of Hydra that is, the release of glutathione from injured cells causes the little animal to engulf food. 

An interesting use is made of addition to a double bond by glutathione-dependent cis-tmns isomerases.76 One of them converts maleate to fumarate with a turnover number of 300 s"1. Similar enzymes, which participate in bacterial breakdown of aromatic compounds (Fig. 25-7), isomerize maleylacetoacetate and maleylpyruvate to the corresponding fumaryl derivatives (Eq. 13-20). The - SH group of bound glutathione is thought to add to the double bond. Rotation can then occur in the enolic intermediate. Thiocyanate ion catalyzes the isomerization of maleic acid nonenzy-matically, presumably by a similar mechanism. 

The gene mutation inhibits hydrolytic cleavage of fumarylacetoacetate into fumarate and acetoacetate. Consequently, the toxic precursors maleylacetoacetate and fumarylacetoacetate accumulate in the liver and kidneys. They possess a reactive double bond and can therefore react with macromolecules to assume the properties of alkylating substances. In addition, intracellular glutathione deficiency develops due to the stable complex formation with glutathione, favouring lipid peroxidations. Enhanced formation of 5-aminolaevulinic acid can also be observed during occasional attacks of acute intermittent porphyria (G. Mitchel et al., 1990). 

Conversion of Homogentisic Add lo Maleylacetoacetic, Fumarylacetoacetic, Fumaric, and Acetoacetic Acids... 

GSTZl Zeta + four allelic liver Activates dichloroacetic acid. Functions as maleylacetoacetate... 

Substrates GSTZl-1— Dichloroacetic acid (conversion to glyoxy-late) and other haloacetic acids, maleylacetoacetate cis-trans isomerase (penultimate step in tyrosine degradation), ethacrynic acid (low). 

Homogentisic acid is further oxidized in another reaction which consumes one molecule of oxygen and requires the presence of both ferrous ions and sulfhy-dryl compounds to yield maleylacetoacetate. The latter compound can be transformed into its isomer in the... 

Homogentisic acid is oxidized by a ferrous ion-requiring enzyme, as shown by Suda and Takeda, and confirmed by several others. Homogentisic oxidase also requires a sulfhydryl compound. One molecule of oxygen is consumed in the reaction, presumably in a single step. The product has been identified as maleylacetoacetate by Knox and Edwards (V). ... 

This enzyme was discovered by Knox and Edwards 221). The cleavage of homogentisic acid yields maleylacetoacetate as would be logically expected. Because of the presence generally of the isomerase, fumaryl-acetoacetate was the product observed by Ravdin and Crandall 220)... 

Formation of maleylacetoacetate upon incubation with homogentisic oxidase is detected by the difference in its absorption spectrum from that of fumarylacetoacetate. The former at pH 3 has an absorption maximum at 230 mu, while the latter compounds has its maximiun at 350 m. At pH 1, fumarylacetoacetate exhibits maximum absorption at 310 m, while maleylacetoacetate has almost no absorption. The difference in spectra is accoimted for by the fact that fumarylacetoacetate exists mainly in the enol form in acid solutions, while maleylacetoacetate is laigely in the keto form. Enol formation is enhanced by borate which leads to the formation of an enol fumarylacetoacetate-borate complex. 

Kynurenine Maleylacetoacetate 4,5-Dimethylcatechol p-Hydroxyacetanilide Formylaspartic acid... 

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