Umbelliferone and Esculetin: Inhibitors or Substrates for Polyphenol Oxidases?

Abstract

Hydroxycoumarins (ar-hydroxy-1,2-benzopyrones) are lactone derivatives of 4-coumaric (p-coumaric) acid. In higher plants these molecules are synthesized via the phenylpropanoid pathway 1) beginning from an unusual hydroxylation in the ortho position of the acid side chain (Chart 1). The next step is the trans/cis isomerization of the double bond of this 2,4-dihydroxycinnamic acid. The subsequent lactonization leads to the formation of 7-hydroxycoumarin, trivial name umbelliferone. Other coumarin derivatives such as 6,7-dihydroxycoumarin (trivial name esculetin) and 7-hydroxy-6-methoxycoumarin (scopoletin) seem to originate from the addition of oxygenated substituents to the aromatic ring of umbelliferone rather than from a cinnamic acid derivative.2) Coumarins are widely distributed in higher plants and are especially abundant in Umbelliferae and Rutaceae families. 3)In the past few years, coumarins received much attention for their diverse bioactivities (reviewed in Borges et al.). 4)Some coumarins from natural sources have been also used as therapeutic agents in humans. 5)Polyphenol oxidase (PPO) is a widespread copper-enzyme, containing two copper ions, that catalyzes the ortho-hydroxylation of monophenols to the corresponding ortho-diphenols (catechols) and the oxidation of catechols to the corresponding ortho-quinones.6) Enzyme nomenclature differentiates between tyrosinase, phenolase, phenol oxidase, polyphenol oxidase, and catechol oxidase, depending on the particular source and also on the authors who have described any particular enzyme. The term tyrosinase is usually adopted for the microorganism, animal, and human enzyme, and refers to the "typical" substrate, tyrosine, while PPO mostly refers to the plant enzyme. PPO is perhaps the most suitable general Recently, an interesting debate arose about the nature (substrate versus inhibitor) of esculetin, a coumarin derivative, for mushroom polyphenol oxidase (PPO). The present study examined the behavior of PPOs preparations from fungal and plant origin towards esculetin as a substrate. Both enzymes were able to oxidize esculetin though at a slow rate. A higher sensitivity was reached when the assay was performed in the presence of 3-methyl-2-benzothiazolinone hydrazone (MBTH) even with a lower amount of PPO. These observations unambiguously confirmed that esculetin has to be considered a substrate for mushroom polyphenol oxidase. The oxidation of esculetin was also demonstrated for the first time by a fungal laccase. This should be taken into account because some mushroom PPO preparations could exert contaminant laccase activity. In addition, a PPO preparation from Ferula communis was demonstrated to use esculetin as a substrate. Umbelliferone, the monophenolic precursor of esculetin along the phenylpropanoid pathway, behaved as a competitive inhibitor for the monophenolase activity of mushroom PPO with a K i value‫410.0؍‬ mM. This is worth a mention because only a few couples of mono-and corresponding o-diphenol show such opposite behavior towards PPO. ...

Metal coordination and tyrosinase inhibition studies with Kojic-βAla-Kojic

“…The side chain rotational freedom is limited by histidine hydrogen bonds (61, 94, 259, 263) to peptide carbonyl oxygen atom as well as phenylalanine wedged between histidines (Phe90 between His94, His259, His296 and Phe292 between His61, His263, His296). Furthermore, His85, is covalently linked via a thioether to Cys83 which fixes the orientation A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 4 of the histidine side chain [10]. The exact role of this post-translationally formed thioether bond remains unclear but a direct role in catalysis can be excluded because it is not conserved [11].…”

“…It is produced as an antibiotic by Aspergillus and Penicillium species and inhibits the enzymatic activity of mushroom tyrosinase [2] (EC 1.14.18.1), an ubiquitous enzyme containing two copper ions. The dinuclear copper center of tyrosinase catalyzes the ortho-hydroxylation of monophenols, oxidation of catechols [3], quinonization of dihydroxycoumarins [4], o-aminophenols and aromatic o-diamines [5; 6].…”

Penentuan Aktivitas Gabungan Ekstrak Etanol Pulosari (Alyxia reinwardtii) dan Secang (Sappan Lignum) Sebagai Inhibitor Tirosinase Yang Potensial Untuk Bahan Kosmetik Melalui Pendekatan In Silico dan In Vitro

“…Beberapa analog kumarin, esculetin, diisolasi dari biji Euphorbialathyris dan menunjukkan seperempat dari aktivitas antitirosinase asam kojat (Masamoto et al 2003). Senyawa analog kumarin baru-baru ini terbukti merupakan substrat tirosinase dari jamur (Sollai et al 2008). Analog kumarin yang lain yaitu 9-hidroksi-4-metoksipsoralen yang diisolasi dari Angelica dahurica terbukti enam kali memiliki aktivitas inhibisi tirosinase dari pada asam kojat (Piao et al 2004).…”

Structure–Activity Relationship Study of Hydroxycoumarins and Mushroom Tyrosinase

“…From our experiments, this latter unnatural compound was not detected at all, indicating that 6-HC exclusively was converted into esculetin, (Figure 3B, line c), which is a substrate for the enzyme. 27 Indeed, after 60 min, the esculetin concentration decreased as it was enzymatically converted to the corresponding o-quinone. It is already known that low concentrations of diphenols can operate by activating the monophenolase activity of tyrosinase.…”