Some Dietary Phenolic Compounds Can Activate Thyroid Peroxidase and Inhibit Lipoxygenase-Preliminary Study in the Model Systems

Abstract

The presented research concerns the triple activity of trans-cinnamic (tCA), ferulic (FA) and syringic acids (SA). They act as thyroid peroxidase (TPO) activators, lipoxygenase (LOX) inhibitors and show antiradical activity. All compounds showed a dose-dependent TPO activatory effect, thus the AC₅₀ value (the concentration resulting in 50% activation) was determined. The tested compounds can be ranked as follows: tCA > FA > SA with AC₅₀ = 0.10, 0.39, 0.69 mM, respectively. Strong synergism was found between FA and SA. The activatory effects of all tested compounds may result from interaction with the TPO allosteric site. It was proposed that conformational change resulting from activator binding to TPO allosteric pocket results from the flexibility of a nearby loop formed by residues Val352-Tyr363. All compounds act as uncompetitive LOX inhibitors. The most effective were tCA and SA, whereas the weakest was FA (IC₅₀ = 0.009 mM and IC₅₀ 0.027 mM, respectively). In all cases, an interaction between the inhibitors carboxylic groups and side-chain atoms of Arg102 and Arg139 in an allosteric pocket of LOX was suggested. FA/tCA and FA/SA acted synergistically, whereas tCA/SA demonstrated antagonism. The highest antiradical activity was found in the case of SA (IC₅₀ = 0.22 mM). FA/tCA and tCA/SA acted synergistically, whereas antagonism was found for the SA/FA mixture.

Keywords: antioxidant activity; dietary polyphenols; inhibition; interactions; isobolographic analysis; lipoxygenase (LOX); thyroid peroxidase (TPO).

Figure 1

Activatory effect of ferulic acid (A), syringic acid (B) and trans-cinnamic acid (C) on thyroid peroxidase TPO activity. Plots are expressed 1/velocity versus 1/guaiacol [μg/mL] without or with activators in a reaction solution.

Figure 2

Molecular interactions of activators with thyroid peroxidase (TPO). (A) Ferulic acid bound to three considered binding pockets. The protein is shown in dark blue cartoon representation, ferulic acid in yellow ball representation and heme in magenta ball representation. The selected pocket is marked with a red square. (B) Ferulic acid, (C) syringic acid and (D) trans-cinnamic acid in a binding pocket of TPO. The protein is shown with cyan carbon atoms in wire representation with the most important residues shown as sticks. Activators depicted with gray carbon atoms in stick representation. Polar bonds are shown as red dashes. Nonpolar hydrogen atoms omitted for clarity.

Figure 3

Lineweaver—Burk double reciprocal plots for the inhibition of lipoxygenase (LOX) by ferulic acid (A), syringic acid (B) and trans-cinnamic acid (C). Plots are expressed 1/velocity versus 1/linoleic acid [μg/mL] without or with inhibitors in a reaction solution.

Figure 4

Molecular interactions of inhibitors with 5-lipoxygenase (5-LOX). (A) trans-cinnamic acid bound to an allosteric binding pocket (general view). The protein is shown in dark blue cartoon representation, trans-cinnamic acid in yellow ball representation and iron ion in magenta ball representation. (B) trans-cinnamic acid, (C) syringic acid and (D) ferulic acid in a binding pocket of 5-LOX. The protein is shown with cyan carbon atoms in wire representation with the most important residues shown as sticks. Inhibitors are depicted with gray carbon atoms in stick representation. Polar bonds are shown as red dashes. Nonpolar hydrogen atoms omitted for clarity.

Figure 5

Dose-normalized isobolograms for chosen phenolic acids with thyroid peroxidase activation activity. (A) trans-cinnamic acid and ferulic acid, (B) syryngic acid and ferulic acid, (C) syringic acid and trans-cinnamic acid).

Figure 6

Dose-normalized isobolograms for chosen phenolic acids with lipoxygenase inhibitory activity. (A) trans-cinnamic acid and syryngic acid, (B) ferulic acid and trans-cinnamic acid, (C) ferulic acid and syryngic acid).

Figure 7

Dose-normalized isobolograms for chosen polyphenols with antiradical activity. (A) syryngic acid and ferulic acid, (B) trans-cinnamic acid and ferulic acid, (C) trans-cinnamic acid and syryngic acid).