Phenylpyruvate tautomerase

The isomerases that catalyze the simplest reactions are tautomerases that promote the oxo-enol (keto-enol) transformation. The widely distributed oxaloacetate tautomerase (Eq. 13-24) is especially active in animal tissues.97120 Oxaloacetate exists to a substantial extent in the enolic form at 38°, 6% enol, 13% oxo, and 81% covalent hydrate.120121 A mammalian phenylpyruvate tautomerase has also been investigated.122... 

Figure 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

Phenylpyruvate tautomerase from beef kidney has been shown to cause preferential loss of the 3-pro-R hydrogen from phenylpyruvic acid using samples of stereospecifically labeled phenylalanine (320). This may explain some unusual 3-pro-R losses (321) in biosynthetic work. 

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