The myocotoxin ochratoxin A is a substrate for phenylalanine hydroxylase in isolated rat hepatocytes and in vivo.

Ochratoxin A (OTA), is a myocotoxin contaminating food and feed stuffs, consisting of a chlorinated dihydroisocoumarin linked through a 7-carboxyl group to L-phenylalanine by an amide bond. When OTA (0.12-1.4 mM) is incubated with freshly isolated rat hepatocytes, it inhibits both the hydroxylation of phenylalanine (0.05 mM) to tyrosine, catalyzed by phenylalanine hydroxylase and the subsequent metabolism of tyrosine as measured by homogentisate oxidation. The IC50 of OTA for phenylalanine hydroxylation is 0.43 mM. OT alpha, (0.5-1.0 mM), the dihydroisocoumarin moiety of OTA, does not inhibit phenylalanine hydroxylase activity under these conditions. During incubations of hepatocytes with uniformly labelled [3H]-OTA and unlabelled phenylalanine, tyrosine-ochratoxin A is formed (up to 6% of the total mycotoxin added), indicating that ochratoxin can act as a substrate for phenylalanine hydroxylase. In vivo tyrosine-OTA is also found in liver of poisoned animals.