title: Umbelliferone and esculetin: inhibitors or substrates for polyphenol oxidases?
abstract: 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) value=0.014 mM. This is worth a mention because only a few couples of mono- and corresponding o-diphenol show such opposite behavior towards PPO. A possible role of PPO in the esculetin fate along biosynthesis pathway of coumarin derivatives is also discussed.
authors.affiliation: Department of Biomedical Science and Technology, University of Cagliari, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Sollai
authors.forename: Paolo
authors.initials: P
authors.lastname: Zucca
authors.forename: Enrico
authors.initials: E
authors.lastname: Sanjust
authors.forename: Daniela
authors.initials: D
authors.lastname: Steri
authors.forename: Antonio
authors.initials: A
authors.lastname: Rescigno
mesh_terms: D000364:Agaricus / Q000201:enzymology; D004156:Catechol Oxidase / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005309:Ferula / Q000737:chemistry; D006027:Glycosides / Q000378:metabolism* / Q000494:pharmacology*; D042845:Laccase / Q000737:chemistry; D010084:Oxidation-Reduction; D020076:Pleurotus / Q000737:chemistry; D011284:Pregnenolone / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D012984:Software; D013056:Spectrophotometry, Ultraviolet; D014468:Umbelliferones / Q000378:metabolism* / Q000494:pharmacology*
chemical_list: D004791:Enzyme Inhibitors; D006027:Glycosides; D014468:Umbelliferones; C081583:esculentin steroid; C031477:7-hydroxycoumarin; D011284:Pregnenolone; D004156:Catechol Oxidase; D042845:Laccase