In enzymology, phenylpyruvate tautomerase or Macrophage migration inhibitory factor (EC 5.3.2.1) is an enzyme that catalyzes the chemical reaction

keto-phenylpyruvate ${\displaystyle \rightleftharpoons }$ enol-phenylpyruvate

Phenylpyruvate tautomerase has also been found to exhibit the same keto-enol tautomerism for 4-Hydroxyphenylpyruvic acid, which is structurally similar to phenylpyruvate but contains an additional hydroxyl moiety in the para position of the aromatic ring.^[1]

This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting keto- and enol-groups. The systematic name of this enzyme class is phenylpyruvate keto---enol-isomerase. This enzyme is also called phenylpyruvic keto-enol isomerase. This enzyme participates in tyrosine metabolism and phenylalanine metabolism.

As of late 2007, 7 structures have been solved for this class of enzymes, with PDB accession codes 1GYJ, 1GYX, 1GYY, 2GDG, 2OOH, 2OOW, and 2OOZ.

• Blasi F, Fragomele F, Covelli I (1969). "Thyroidal phenylpyruvate tautomerase. Isolation and characterization" (PDF). J. Biol. Chem. 244 (18): 4864–70. doi:10.1016/S0021-9258(18)94283-3. PMID 5824560.
• Knox WE (1955). [38] Enzymes involved in conversion of tyrosine to acetoacetate. Methods Enzymol. Vol. 2. pp. 289–295. doi:10.1016/s0076-6879(55)02202-7. ISBN 978-0-12-181802-9.
• KNOX WE, PITT BM (1957). "Enzymic catalysis of the keto-enol tautomerization of phenylpyruvic acids". J. Biol. Chem. 225 (2): 675–88. doi:10.1016/S0021-9258(18)64866-5. PMID 13416270.