TUESDAY, Oct. 2 (HealthDay News) -- The cooperative binding of acetaminophen and caffeine in the P450 3A4 enzyme disrupts the coordination of acetaminophen to the heme iron, enhancing the rate of oxidation of acetaminophen to its toxic metabolite, N-acetyl-p-benzoquinone (NAPQI), according to a study published online Sept. 26 in Chemical Research in Toxicology.
Sidney D. Nelson, Ph.D., of the University of Washington in Seattle, and colleagues expressed human P450 3A4 in Escherichia coli, which was purified, and reconstituted using artificial liposomes. The researchers used NMR T1 paramagnetic relaxation techniques to calculate distances from the P450 3A4 heme iron to protons of acetaminophen alone and in the presence of caffeine.
The investigators found that both acetaminophen and caffeine bound at the active site of P450 3A4. The binding of caffeine disrupted the coordination of acetaminophen to the heme iron of P450 3A4, enhancing the rate of formation of its toxic metabolite.
"This disruption resulted in changes in the acetaminophen orientation within the active site and decreased the homotropic allosterism exhibited by acetaminophen. The altered positioning of one substrate by a second substrate/effector shown in this work is a likely explanation for much of the allosterism reported for P450 3A4."