Tuchman M, Ramnaraine M L, O'Dea R F
Cancer Res. 1985 Nov;45(11 Pt 1):5553-6.
The kinetic properties and control mechanisms of 5-fluorouracil (5-FU), uracil, and thymine degradation by rat liver dihydropyrimidine dehydrogenase were studied in vitro. The calculated Michaelis constant (Km) for 5-FU was 3.49 +/- 0.41 (SE) microM, similar to those for uracil (2.26 +/- 0.28 microM) and for thymine (2.23 +/- 0.34 microM). However, the reduction of 5-FU appears to be most sensitive to the inhibitory effects of increased substrate concentration. The specific activities of dihydropyrimidine dehydrogenase (nmol/min/mg of protein) for 5-FU, uracil, and thymine were 0.82, 0.68, and 0.56, respectively. Uridine was found to be a potent noncompetitive inhibitor of pyrimidine base degradation in vitro, displaying an inhibition constant (Ki) for 5-FU of 0.71 microM. Total inhibition of 5-FU degradation occurred at a uridine concentration of 10 microM, whereas thymidine was found to be a much less potent noncompetitive inhibitor of pyrimidine base degradation (Ki 24 microM). This paper provides the first documentation of in vitro inhibition of dihydropyrimidine dehydrogenase activity by nucleosides. The concomitant utilization of uridine and 5-FU in clinical situations might prove useful by decreasing 5-FU catabolism to toxic metabolites as well as enhancing 5-FU cytotoxicity.
在体外研究了大鼠肝脏二氢嘧啶脱氢酶对5-氟尿嘧啶（5-FU）、尿嘧啶和胸腺嘧啶的降解动力学特性及调控机制。计算得出5-FU的米氏常数（Km）为3.49±0.41（SE）μM，与尿嘧啶（2.26±0.28μM）和胸腺嘧啶（2.23±0.34μM）的米氏常数相似。然而，5-FU的还原似乎对底物浓度增加的抑制作用最为敏感。二氢嘧啶脱氢酶对5-FU、尿嘧啶和胸腺嘧啶的比活性（nmol/分钟/毫克蛋白质）分别为0.82、0.68和0.56。发现尿苷是体外嘧啶碱基降解的有效非竞争性抑制剂，对5-FU的抑制常数（Ki）为0.71μM。在尿苷浓度为10μM时，5-FU降解完全被抑制，而胸苷被发现是嘧啶碱基降解的效力低得多的非竞争性抑制剂（Ki为24μM）。本文首次记录了核苷对二氢嘧啶脱氢酶活性的体外抑制作用。在临床情况下同时使用尿苷和5-FU可能会通过减少5-FU分解代谢为有毒代谢物以及增强5-FU细胞毒性而证明是有用的。
