Redox reaction between amino‐(3,4‐dihydroxyphenyl)methyl phosphonic acid and dopaquinone is responsible for the apparent inhibitory effect on tyrosinase

Abstract

Amino-(3,4-dihydroxyphenyl)methyl phosphonic acid, the phosphonic analog of 3,4-dihydroxyphenylglycine, had been previously reported as a potent inhibitor of tyrosinase. The mechanism of the apparent enzyme inhibition by this compound has now been established. Amino-(3,4-dihydroxyphenyl)methyl phosphonic acid turned out to be a substrate and was oxidized to o-quinone, which evolved to a final product identified as 3,4-dihydroxybenzaldehyde, the same as for 3,4-dihydroxyphenylglycine. Monohydroxylated compounds (amino-(3-hydroxyphenyl)methyl phosphonic acid and amino-(4-hydroxyphenyl)methyl phosphonic acid) were not oxidized, neither was 4-hydroxy-L-phenylglycine. However, the relatively high K m for amino-(3,4-dihydroxyphenyl)methyl phosphonic acid (0.52 mM) indicated that competitive inhibition could not entirely explain the previously reported strong inhibitory effect (K i ¼ 50 and 97 lM for tyrosine and 3-(3,4-dihydroxyphenyl)alanine (Dopa) as substrates, respectively). Neither was the enzyme covalently inactivated to a significant degree. Spectroscopic and electrochemical analysis of the oxidation of a mixture of Dopa and the inhibitor demonstrated that the phosphonic compound reduced dopaquinone back to Dopa, thus diminishing and delaying the formation of dopachrome. This produces an apparent strong inhibitory effect when the reaction is monitored spectrophotometrically at 475 nm. In this peculiar case Dopa acts as a redox shuttle mediating the oxidation of the shorter phosphonic homolog. Decomposition of the phosphonic o-quinone to 3,4-dihydroxybenzaldehyde drives the reaction against the slightly unfavorable difference in redox potentials.Keywords: tyrosinase; redox exchange; quinone; phosphonic amino acids; 3,4-dihydroxybenzaldehyde.Tyrosinase (EC 1.14.18.1) is a copper-containing enzyme widely distributed in nature. It catalyses the hydroxylation of monophenols to o-diphenols and the oxidation of the latter to o-quinones using molecular oxygen. Its action on the physiological substrate, L-tyrosine, produces L-3-(3,4-dihydroxyphenyl)alanine (L-Dopa) and then dopaquinone, which undergoes a series of nonenzymatic reactions leading to melanins [1]. The enzyme is responsible for melanization in animals and browning in plants. As browning in food products is an undesirable process, there has been a constant need in food industry for compounds preventing this reaction. Inhibition of mammalian tyrosinase has also been indicated as a possible approach to control human melanoma [2]. Although a large number of tyrosinase inhibitors have been described in the literature [3], the search for new natural products and synthetic compounds with such activity still continues [4]. Some of the most potent, competitive inhibitors include mimosine [5,6], tropolone [7,8], and kojic acid [9][10][11][12]. Some of us had previously shown that amino-(3,4-dihydroxyphenyl)methyl phosphonic acid, the phosphonic analog of 3,4-dihydroxyphenylglycine, was also a potent inhibitor of tyrosinase [13]. With a K i of 50 and 97 lM for tyrosin...

Phosphonic and Phosphinic Acid Derivatives as Novel Tyrosinase Inhibitors: Kinetic Studies and Molecular Docking

“…Most investigations on interactions of phosphonic acid derivatives with tyrosinase reported in the literature [38][39][40] regards structures which are analogs of tyrosinase natural substrates: L-Tyrosine and L-DOPA and they contain phenolic or diphenolic moiety. Some of the compounds acted as tyrosinase new substrates and some revealed moderate inhibitory activity.…”

Electrochemical interfacial adsorption mechanism of polyphenolic molecules onto Hanging Mercury Drop Electrode surface (HMDE)

“…Also, HMDE was previously used to study the mechanism of the reduction and breaking of the S-S bonds in proteins [25,26], dithiocarbamate pesticides [27] and cysteine [28,29], using Cathodic Stripping Voltammetry. In previous work [30], HMDE has been employed for the analytical determination of the redox states of phosphonic acid derivatives, amino-(4-dihydroxyphenyl) methyl phosphonic acid, and 3-(3,4-dihydroxyphenyl)-alanine (Dopa) by cyclic voltammetry (CV). The theoretical electrochemical aspects of Cathodic Stripping Voltammetry (CSV) have been applied for the adsorption of monophenols [30] and polyphenols [11,32,33] on a HMDE and a glassy carbon mini-electrode respectively.…”

Dialkylphosphorylhydrazones as potent tyrosinase inhibitors

“…Among these, (3,4-dihydroxyphenyl)methylamino phosphonic acid has shown the higher inhibitory potential. 15,16 Thus, in continuation of our research dealing with the synthesis of new organophosphorus compounds and their potential biological activities, we have screened nineteen new dialkylphosphorylhydrazones, whose syntheses are described elsewhere, 17 (Figure 1) for their tyrosinase inhibitory potential.…”