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

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Regional mapping of the gene encoding dihydroorotate dehydrogenase, an enzyme involved in UMP synthesis, electron transport, and superoxide generation, to human chromosome region 16q22

De novo UMP synthesis is a critical metabolic pathway for nucleic acid synthesis and for a variety of metabolic pathways. The pathway is a target for many widely used cancer chemotherapy agents, several of which are pyrimidine analogs. Humans and cattle have been described with mutations in UMP synthesis that lead to serious inborn errors of metabolism. Dihydroorotate dehydrogenase (EC 1.3.3.1) (DHODH) carries out the fourth committed step in the pathway and may also be important for mitochondrial electron transport and oxygen radical metabolism. We report here that the gene encoding this enzyme in humans is located in the chromosomal region 16q22. With the mapping of DHODH, the mapping of all the steps of UMP synthesis is complete. All three genes involved map to different human chromosomes. This information is important in consideration of regulation of UMP synthesis in mammals, including humans.

Lesch-Nyhan syndrome, caffeine model: increase of purine and pyrimidine enzymes in rat brain

Rats ingesting high doses of caffeine reproduce the self-destructive behaviour observed in the Lesch-Nyhan syndrome. This syndrome includes a deficit in hypoxanthine-guanine phosphoribosyltransferase. We have observed, however, that the activity of hypoxanthine-guanine phosphoribosyltransferase increases in direct proportion to the concentration of caffeine found in rat brain. It appears, therefore, that the caffeine model is not a true model for the Lesch-Nyhan syndrome, or alternatively, that the deficit in hypoxanthine-guanine phosphoribosyltransferase is coincidental and not a main key to the multifarious aspects of the syndrome, particularly the self-mutilation. The possibility that levels of dopamine are increased in the caffeine model are discussed as a basis for the destructive behaviour. We have found also that ingestion of large amounts of caffeine increases the activities in rat brain of adenosine deaminase, purine nucleoside phosphorylase, aspartate carbamoyl-transferase, dihydroorotase, and dihydroorotate oxidase.

Phase-shifting of cycles in level of serotonin N-acetyltransferase activity and cyclic GMP content of cultured chick pineal glands by methotrexate

Methotrexate at 1 microM stimulated increase of serotonin N-acetyltransferase (NAT) activity in chick pineal glands cultured under each of three conditions of illumination. The peak of the circadian rhythm in NAT activity and the "spike" in content of cyclic GMP were both advanced in pineal glands cultured in the dark from midphotoperiod. In contrast, the time of peak NAT activity in glands cultured in the dark from late photoperiod was unaffected. In addition, methotrexate did not affect times of reaching maximum NAT activities in glands cultured from midphotoperiod in the light or under diurnal illumination. Doubling the concentration of methotrexate also eliminated the lag phase in increase of NAT activity in glands cultured in the dark. However, at a concentration of 5 microM methotrexate the curve depicting increase of NAT activity was biphasic, and neither time nor level of peak NAT activity differed from those of control glands. Results of attempts to demonstrate persistent effects of exposure to methotrexate were inconclusive.