Description_x000D_ General description_x000D_ 4-Hydroxyphenylpyruvate Dioxygenase (4-HPPD) is a key enzyme in the catabolism of tyrosine and phenylalanine. 4-HPPD catalyzes the synthesis of homogenistate (2,5-dihydroxyphenylacetate) from ρ-hydroxyphenylpyruvate and molecular oxygen. 4-HPPD also catalyzes the conversion of α-ketoisocaproate to β-hydroxyisovalerate. 4-HPPD requires Fe2+ as an essential cofactor._x000D_ Rat 4-HPPD has a minimum of 80% homology with human 4-HPPD. Rat 4-HPPD is expressed abundantly in hepatocytes. It is also expressed in kidney, particularly in epithelial cells of the proximal tubules and the distant tubules. 4-HPPD has been identified as a potential biomarker of drug-induced liver injury in rat studies._x000D_ Unit Definition_x000D_ One unit will oxidize 1 μmole of molecular oxygen per minute at pH 7.0, 25 °C, in the presence of 4-hydroxyphenylpyruvate.