Thymidylate synthase from untreated human colorectal cancer and colonic mucosa: enzyme activity and inhibition by 5-fluoro-2'-deoxy-uridine-5'-monophosphate.

Inhibition of thymidylate synthase (TS) by the 5-fluorouracil (5-FU) metabolite FdUMP is considered to be the main mechanism of action of 5-FU. TS from colorectal tumours and normal colon mucosa from 10 untreated patients was studied. There was a large variation in the activity of tumour TS both at 1 and 10 mumol/l of its substrate dUMP; in normal mucosa this variation was less. Inhibition by 10 nmol/l FdUMP in tumours varied from 80 to 90% at 1 mumol/l dUMP; in normal mucosa, inhibition varied from 10 to 80%. The number of FdUMP binding sites ranged from 0.1 to 1 in tumours but such binding sites were not detectable in normal mucosa. The ratio between TS activity and FdUMP binding sites varied considerably in tumours but not in normal mucosa. The deviations from normal kinetics may represent a mutant TS form. Alterations in TS may partly account for differences in response to 5-FU.