The interaction and inhibition of muscle lactate dehydrogenase by the alkaloid caffeine

Kinetic analysis showed that the alkaloid caffeine is a competitive inhibitor of the enzyme lactate dehydrogenase with respect to substrate pyruvate, and a non-competitive inhibitor with respect to the coenzyme NADH. The inhibitor constant Ki is 0,54 mM. Scatchard analysis determined the dissociation constant for a single independent binding site of the ternary lactate dehydrogenase - NADH - caffeine complex (KE-NADH-CAFFEINE) and the number of binding sites to be 0,14 mM and 3,83 respectively. Caffeine binds to a hydrophobic domain in the substrate binding site. Alternate nucleophilic - electrophilic functionalities within the inhibitor molecule are proposed to be the fundamental reason for the inhibition.