Inhibition of Peroxidase Activity by Methylxanthines

Peroxidase (POD) is an important antioxidant enzyme that catalyzes oxidation of a number of organic and non-organic substrates using hydrogen peroxide as the electron acceptor. At physiological low levels, reactive oxygen species (ROS) can act as redox messengers in the regulation of intracellular signaling. However, in excess amounts they can suppress the immune system and cause oxidative stress. Considering the high consumption of tea and coffee as the most common drink in the world, in the present study the effect of caffeine and theophylline on the activity of POD has been investigated. The activity of POD was measured by following absorption at 510 nm due to the oxidation of 4-aminoantipyrine in the absence and presence of caffeine and theophylline. The enzyme kinetic parameters were then measured and compared in each case. It was shown that both methylxanthines acted as inhibitors with IC50' the amount of inhibitor to reduce the enzyme activity by 50%, of 0.6 and mM 0.55 mM for caffeine and theophylline respectively. The kinetic constants, Km and Vmax' indicated that both inhibitors worked by an un-competitive mechanism on POD activity. The values of Ki were calculated as 0.08 and 0.045 mM for caffeine and theophylline respectively. Lower values of IC50 and Ki for theophylline compared to caffeine, led us to a final conclusion that theophylline is a stronger inhibitor of POD than caffeine.