P-Hydroxyphenyl pyruvate

It is fortunate that Medes was available to make such a complete and able investigation, as no further eases of tyrosinosis have been reported and the metabolic defect must therefore be exceedingly rare. (There having been only one case recorded nothing is, of course, known of the genetics of its inheritance). Recently, however, other cases of p-hydroxyphenyl-pyruvic acid excretion have been observed, and though these are probably of a different type it is convenient to consider them here. 

Knox (483) has purified 100-fold an enzyme converting p-hydroxyphenyl-pyruvate to homogentisate. The system requires either ascorbic acid or dichlorophenolindophenol, and appears to be much more active than systems previously reported. The conversion occurs in one step. Details of the reaction are awaited with interest especially as 2,5-dihydroxyphenyl-pyruvate appears to be neither an intermediate nor an inhibitor (209a). 

Abbreviations used are GSH, glutathione ACTH, adrenocorticotropic hormone DPN and TPN, di- and triphosphopyridine nucleotides pHPP, p-hydroxyphenyl-pyruvate and DHA, dehydroascorhic acid. 

The oxidative decarboxylation of prephenic acid to form p-hydroxyphenyl pyruvate is similar, in that it is coupled to an NAD+-linked oxidation (Scheme V). However, there are several important differences. In the first place, this enzyme... 

Recently extracts of acetone powder and homogenates of liver have been obtained which specifically oxidize tyrosine, p-hydroxyphenyl-pyruvic acid, and homogentisic acid to acetoacetic acid. The reaction is aerobic and requires the uptake of 4 atoms of oxygen per molecule for the oxidation of either L-tyrosine or p-hydroxyphenylpyruvic acid to yield 1 molecule of CO2 and 1 of acetoacetic acid oxidation of homogentisic acid to acetoacetate requires the uptake of 2 atoms of oxygen. 

Pyruvate inhibits the oxidation of tyrosine and p-hydroxyphenyl-pyruvate, but not of homogentisic acid. This inhibition can be reversed by ascorbic acid or hydroquinone and its site of action, then, appears to be in relation to 2,5-dihydroxyphenyl formation from p-hydroxyphenyl-pyruvate. 

Excess substrate inhibited the oxidation of the p-hydroxyphenyl-pyruvate as did also benzoquinone acetic acid. This could be prevented by adding large amounts of ascorbic acid or small amounts of reduced 2,6-dichlorophenolindophenol. The latter was seven hundred times as effective as ascorbic acid with the purified enzyme 214), Zannoni and LaDu speculate that a product formed from p-hydroxyphenylpyruvate is the true inhibiting agent of the reaction. 

Dihydroxyphenylpyruvate was not oxidized to homogentisate, thus it cannot be an intermediate in the oxidation of p-hydroxyphenyl-pyruvate. The results suggest that the hydroxylation shift of the side chain and decarboxylation of the p-hydroxyphenylpyruvate are simultaneous processes. Additional evidence that 2,5-dihydroxyphenylpyruvate is not the intermediate was obtained by experiments in which the relative rates of oxidation of this compound and of p-hydroxyphenylpyruvate were compared in homogenates of rat liver, where the reaction proceeded to formation of acetoacetic acid. Oxidation of p-hydroxyphenylpyruvate proceeded much more rapidly. Other analogs of p-hydroxyphenylpyruvate were found to be inactive as substrates. 

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