The mechanism of inhibitory effect of caffeine from Iranian tea on sigmoidal kinetics of adenosine deaminase enzyme activity

Caffeine is one of most important ligand of adenosine deaminase (ADA). In the present study, changes in adenosine deaminase activity were measured in Tris-HCl buffer 50mM, pH 7.3 at 37oC in the presence and absence of caffeine in increasing concentrations of adenosine as a substrate (0-50µM) and in various incubation times with caffeine. Regarding the analysis of the saturation curves, for the first time, their sigmoidal shape shows considerable differences with previous reports which can be referred to differences in concentration range of substrate and incubation time with caffeine. Based on the sigmoidal shape of saturation curve, mechanism of caffeine effect is explainable.  Moreover, based on finding of this study, the effect of caffeine on ADA activity is a function of incubation time and will be stable after 6 hours incubation of enzyme with caffeine. This is noteworthy when interpreting the results of the studies on ADA and its ligands.