Mechanism for the reactivation of the peroxidase activity of human cyclooxygenases: investigation using phenol as a reducing cosubstrate.

It has been known for many years that the peroxidase activity of cyclooxygenase 1 and 2 (COX-1 and COX-2) can be reactivated in vitro by the presence of phenol, which serves as a reducing compound, but the underlying mechanism is still poorly understood. In the present study, we use phenol as a model compound to investigate the mechanism by which the peroxidase activity of human COXs is reactivated after each catalytic cycle. Molecular docking and quantum mechanics calculations are carried out to probe the interaction of phenol with the peroxidase site of COXs and the reactivation mechanism. It is found that the oxygen atom associated with the Fe ion in the heme group (i.e., the complex of Fe ion and porphyrin) of COXs can be removed by addition of two protons. Following its removal, phenol can readily bind inside the peroxidase active sites of the COX enzymes, and directly interact with Fe in heme to facilitate electron transfer from phenol to heme. This investigation provides theoretical evidence for several intermediates formed in the COX peroxidase reactivation cycle, thereby unveiling mechanistic details that would aid in future rational design of drugs that target the peroxidase site.

Ellagic acid, a plant phenolic compound, activates cyclooxygenase-mediated prostaglandin production.

In recent years, ellagic acid (EA), a naturally-occurring phenolic compound richly contained in some of the human food sources such as Longan and Litchi, was reported to have a number of biological effects. Based on our earlier 3D-QSAR/CoMFA models for cyclooxygenase (COX) I and II, we hypothesize that EA may have the potential to modulate the catalytic activity of COX enzymes, and this hypothesis is examined in the present study. The results from both in vitro and in vivo experiments show that EA is an activator of COX enzyme-catalyzed production of prostaglandin E2, a representative prostaglandin tested. Mechanistically, EA can activate the peroxidase active site of COX enzymes by serving as a co-substrate, presumably for the reduction of protoporphorin IX with FeIV inside. The effect of EA is abrogated by the co-presence of galangin, which is known to bind to COX's peroxidase active site and thereby blocks the effect of the reducing co-substrates. In view of the known physiological functions of COX enzymes in the body, it is suggested that some of the pharmacological and/or toxicological effects of EA may result from an increased production of certain prostaglandins and their related derivatives in the body.

Stimulation of the Production of Prostaglandin E2 by Ethyl Gallate, a Natural Phenolic Compound Richly Contained in Longan.

Ethyl gallate is a phenolic compound richly contained in Longan. In traditional Chinese medicine, Longan is widely known as a fruit with "hot" properties, with a tendency to promote inflammatory and certain other responses. The mechanism for its proinflammatory as well as health beneficial effects is poorly understood. Based on our earlier observation that certain natural phenolic compounds can serve as reducing cosubstrates for cyclooxygenases (COXs), we sought to test a hypothesis that ethyl gallate may activate the catalytic activity of the COX enzymes. Results from studies using cultured cells and animals show that ethyl gallate can activate the production of prostaglandin E2, a representative prostaglandin tested in this study. Computational analysis indicates that ethyl gallate can activate the peroxidase active sites of COX-1 and COX-2 by serving as a reducing cosubstrate. The effect of ethyl gallate is abrogated by galangin, which is known to bind to the same peroxidase active sites of COX-1 and COX-2 as a competitive inhibitor. The findings of this study offer support for a novel hypothesis that the proinflammatory as well as health beneficial effects of Longan may partly attributable to the activation of COX-1 and COX-2 by ethyl gallate.