Effects of caffeine on pyrimidine biosynthesis and 5-phosphoribosyl 1-pyrophosphate metabolism in Chinese hamster cells.

Caffeine, at doses which enhance killing by UV light, inhibits the biosynthesis of pyrimidines in Chinese hamster ovary cells (K1) in culture. This inhibition was measured as a decrease in [14C]UTP and [14C]CTP accumulation after a 3-h incubation with [14C]aspartate or [14C]orotate and a similar decrease in Urd-A cells (which lack the first three enzymes of the pathway) using [14C]orotate as substrate. There was no such inhibition in Urd-C cells (which lack the last two enzymes) using [14C]aspartate as substrate and measuring accumulation of orotate. There is some inhibition of the fifth enzyme of the pathway, orotate phosphoribosyltransferase by caffeine in vitro and this is most striking at low 5-phosphoribosyl 1-pyrophosphate concentrations. The level of 5-phosphoribosyl 1-pyrophosphate is decreased in Chinese hamster ovary K1 cells by about 20% after 3 h and by about 70% after 16 h in the presence of caffeine. It is suggested that inhibition of pyrimidine biosynthesis by caffeine over a 16-h period may be due mainly to decreased intracellular 5-phosphoribosyl 1-pyrophosphate levels but that in the decrease in pyrimidine accumulation over 3 h, direct inhibition of orotate phosphoribosyltransferase by caffeine may also play a role.