Nitric Oxide: The Key Molecule for Polyphenols Antimicrobial Action

Abstract

The role of hydroxycinnamic acids as antioxidants, in vivo, has been widely discussed, but, recently, a great debate has focused on their antimicrobial action. In general, for the hydroxycinnamic acids' action, the presence of NO, which is known to be an antimicrobial agent, seems compulsory; its production goes through the intermediacy of the nitrosonium ion, and a very low pH, for instance, as in the stomach, is requested. However, the action of the hydroxycinnamic acids seems to take place even in different biological compartments, i.e., characterized by different pHs and conditions, and then, for NO production, an alternative mechanism could be involved. In this light, evidence for the NO formation, via an E.T. mechanism, even in mildly acidic conditions (pH = 6.4), was obtained by reacting an aqueous buffer solution of acidic nitrite (HNO 2 ) with the hydroxycinnamic acids ferulic, caffeic, p-coumaric and sinapic. Experiments conducted by EPR spectroscopy, let to detect the NO formation, and the efficiency of the process depending on the available amount of free polyphenol, and the intrinsic nature of the hydroxycinnamic acids. Thus, the production of NO through a non-enzymatic mechanism, in light acidic conditions, would account for the antimicrobial action of hydroxycinnamic acids, even in unconventional biological compartments, and for NO as the key-molecule.

The crucial role of non-enzymatic NO-production in plants. An EPR study

“…So, since different is the reducing strength, different will be the efficiency in inducing the release of NO from the acid nitrite, and then the amount of NO trapped. In support of this, previous experiments conducted on hydroxycinnamic acids (Grossi and Casadei, 2020) such as ferulic acid (7), caffeic (8), and p-coumaric (9) (see Table 1), have shown the ability to induce the release of NO, through an E.T. process, from a solution of acid nitrite.…”

“…This procedure had been previously used to detect NO in plants but, in those experiments, the spin-trap was directly introduced into the reaction pot (Caro andPuntarulo, 1999: Jasid, et al, 2006;Corpas, et al, 2004), and this changing the focal point of the process due to the reducing action of the trap (iron Fe ++ ), which will contribute to the release of NO. To avoid this, we conducted our EPR experiments with aqueous solutions of HCAAs and an acid nitrite buffer solution (pH = 6.4), keeping the spin trap solution separate, i.e., in a different vial as described in the experimental section (Qiao et al, 2008;Grossi and Casadei, 2020;Cevahir et al, 2007;Choi et al, 2007;Takahashi et al, 2003). Table 1.…”

“…Finally, it was necessary to verify the role of hydroxycinnamic radicals. Results previously obtained in our laboratory (Grossi and Casadei, 2020) on hydroxycinnamic acids, such as caffeic (7), ferulic (8), and p-coumaric (9) could be used as a reference, allowing a comparison with putrescine-amides 1, 2, and 3, under the same experimental conditions. The spectra we obtained, plotted on the same graph, showed amides to have a greater ability to influence the release of nitric oxide than pure hydroxycinnamic acids, Figure 5.…”