TISSUE DISTRIBUTION OF TRO-4-TRIFLUOROMETHYLBENZOYL)CYCLOHEXANE-1,3-DIONE (NTBC) - EFFECT ON ENZYMES INVOLVED IN TYROSINE CATABOLISM AND RELEVANCE TO OCULAR TOXICITY IN THE RAT

Administration of a single oral dose of tro-4-trifluoromethylbenzoyl)c yclohexane-1,3-dione (NTBC) to rats produced a marked tyrosinemia in t he plasma and aqueous humor. The tyrosinemia was both time- and dose-d ependent with the duration being more marked at the higher doses. The dose-response curve was very steep with a single dose of 1.5 mu mol NT BC/kg (0.5 mg/kg) and above producing maximal concentrations of tyrosi ne in plasma of about 2500 nmol/ml and in aqueous humor of about 3500- 4000 nmol/ml at 24 hr after dosing. Analysis of the key hepatic enzyme s involved in tyrosine catabolism showed that 4-hydroxyphenylpyruvate dioxygenase (HPPD) was markedly inhibited soon after dosing at either 0.3 or 30 mu mol/kg (0.1 or 10 mg/kg) NTBC and that the activity recov ered very slowly. In response to the tyrosinemia, the activity of tyro sine aminotransferase (TAT) in the liver was induced about twofold, wh ile the activity of homogentisic acid oxidase (HGO) was not affected. Daily oral administration of NTBC for 6 weeks induced lesions to the c ornea of the eye, with a dose of 0.3 mu mol/kg/day producing about a 3 8% incidence and a higher dose of 30 mu mol/kg/day a 75% incidence. Ad ministration of a single oral dose of [C-14]NTBC at either 0.3 or 30 m u mol/kg led to selective retention of radiolabel in the liver and to a lesser extent in the kidneys and the Harderian gland. Concentrations of radioactivity in the liver and kidneys remained constant over 4 da ys and after the lower NTBC dose were about 2 nmol/g wet wt and 0.9 nm ol/g wet wt, respectively. Subcellular fractionation of the liver show ed that the majority of the radiolabel, >90%, was associated reversibl y with the cytosol fraction. No retention of radiolabel was detected i n the cornea, the site of toxicity. Our studies indicate that NTBC bin ds to protein in rat liver cytosol, inhibits the hepatic cytosolic enz yme HPPD, and causes a marked and sustained tyrosinemia. We suggest th at this marked and sustained ocular tyrosinemia produced by NTBC in th e rat is responsible for the corneal lesions since similar corneal les ions are produced by feeding rats a high tyrosine diet. (C) 1996 Acade mic Press, Inc.