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Free Radic Biol Med. 2016 Aug;97:148-157. doi: 10.1016/j.freeradbiomed.2016.05.019. Epub 2016 May 19.

Quinone-induced protein modifications: Kinetic preference for reaction of 1,2-benzoquinones with thiol groups in proteins.

Author information

1
School of Food Science and Technology, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, China; Department of Food Science, Faculty of Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg, Denmark; Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, South China University of Technology, Guangzhou, China. Electronic address: yli.scut@gmail.com.
2
Department of Food Science, Faculty of Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg, Denmark. Electronic address: Jongberg@food.ku.dk.
3
Department of Food Science, Faculty of Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg, Denmark. Electronic address: mola@food.ku.dk.
4
Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen, Denmark. Electronic address: davies@sund.ku.dk.
5
Department of Food Science, Faculty of Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg, Denmark; Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen, Denmark. Electronic address: mnl@food.ku.dk.

Abstract

Oxidation of polyphenols to quinones serves as an antioxidative mechanism, but the resulting quinones may induce damage to proteins as they react through a Michael addition with nucleophilic groups, such as thiols and amines to give protein adducts. In this study, rate constants for the reaction of 4-methylbenzoquinone (4MBQ) with proteins, thiol and amine compounds were determined under pseudo first-order conditions by UV-vis stopped-flow spectrophotometry. The chemical structures of the adducts were identified by LC-ESI-MS/MS. Proteins with free thiols were rapidly modified by 4MBQ with apparent second order rate constants, k2 of (3.1±0.2)×10(4)M(-1)s(-1) for bovine serum albumin (BSA) and (4.8±0.2)×10(3)M(-1)s(-1) for human serum albumin at pH 7.0. These values are at least 12-fold greater than that for α-lactalbumin (4.0±0.2)×10(2)M(-1)s(-1), which does not contain any free thiols. Reaction of Cys-34 of BSA with N-ethylmaleimide reduced the thiol concentration by ~59%, which resulted in a decrease in k2 by a similar percentage, consistent with rapid adduction at Cys-34. Reaction of 4MBQ with amines (Gly, Nα-acetyl-l-Lys, Nε-acetyl-l-Lys and l-Lys) and the guanidine group of Nα-acetyl-l-Arg was at least 5×10(5) slower than with low-molecular-mass thiols (l-Cys, Nα-acetyl-l-Cys, glutathione). The thiol-quinone interactions formed colorless thiol-phenol products via an intermediate adduct, while the amine-quinone interactions generated colored amine-quinone products that require oxygen involvement. These data provide strong evidence for rapid modification of protein thiols by quinone species which may be of considerable significance for biological and food systems.

KEYWORDS:

Plant phenols; Protein modification; Quinone-protein adduct; Rate constants; Thiols

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