BIOCHEMICAL ACTION OF PLANT METHYLXANTHINES

The many pharmacological and biochemical effects of caffeine may be explained in part by its inhibitory action in vitro and in vitro upon enzymes which metabolize purines. We have demonstrated that in Crithidia fasciculata methylxanthine, as well as theophylline, are rather weak competitive inhibitors of adenine aminohydrolase, ribonucleoside hydrolase, hypoxanthine and guanine phosphoribosyltransferases. Although caffeine does not interfere with purine base transport in Crithidia, in leucocytes purine uptake is reduced. While the methylxanthines are weak purine enzyme inhibitors, the large number of enzymes affected accounts for their physiological importance in these cells.