This dissertation, "Protective effects of natural polyphenols in reactive carbonyl species/lipid peroxidation-induced toxicity" by Qin, Zhu, 朱芹, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Oxidative degradation of lipids, not only leads to the quality deterioration in foodstuffs but also associates with a multitude of physiological processes. One of the causations involved in these damaging effects is the generation of reactive carbonyl species (RCS) in lipid peroxidation process. RCS are notorious toxins that possess reactivity towards biological nucleophiles (such as proteins and DNA) with potential functional alternation in these biomolecules. Therefore, the exogenous intervention is required to inhibit the toxicity related to RCS/lipid peroxidation. In present study, the screening for effective natural polyphenols to trap two representative RCS, acrolein (ACR) and 4-hydroxy-trans-2-nonenal (HNE), was performed with mechanism elucidation. It was found that the polyphenols from the categories of flavan-3-ols, theaflavins, cyanomaclurins and dihydrochalcones were effective scavengers of ACR/HNE. Subsequently, facilitated by HPLC, LC-MS/MS and NMR analysis, the characterization of polyphenols' as sacrificial nucleophiles towards these two electrophiles products was accomplished. Michael addition at A ring of polyphenols' to the C=C bond of ACR/HNE was suggested to be responsible for trapping of these two RCS and thus render their active sites unavailable to covalently modify critical biomolecules. Further investigation of phloretin's effect to attenuate ACR-induced modification on lysine residue and proteins was carried out. Phloretin's protective effect against ACR's toxicity was clearly reflected by its inhibition of the formation of Nε-(3-formyl-3,4-dehydropiperidino) lysine (FDP-lysine), blocking the electrophilic site in FDP-lysine, lowering protein carbonylation in bovine serum albumin (BSA) and lessening protein oligomerization in bovine pancreas ribonuclease A. Such protection might be mediated by phloretin's directly trapping of ACR and consequently deactivation of ACR in covalent modification of amino acids and proteins. The biological relevance of polyphenols' trapping activity of ACR was explored in a cell culture model. Natural polyphenols including phloretin, EGCG and quercetin were proved to be active to inhibit ACR-induced cytotoxicity in human neuroblastoma SH-SY5Y cells. The cytoprotection of phloretin (as the most potent one in alleviation of ACR stress) was suggested to be achieved through the reduction of the increased cellular protein carbonyl level as revealed by Western blotting analysis. In the final part of this study, an in vitro system containing metal-catalyzed fatty acids and BSA was established to study the modification on protein induced by lipid peroxidation and possible inhibitory effects conferred by some natural polyphenols. The protective effects of these polyphenols against lipid peroxidation-induced modification on BSA was manifested by the observation of reduced levels of high-molecular-weight proteins, MDA-related fluorescent substances and protein carbonylation. However, poor correlations were found between such protection and antioxidant activities, suggesting alternative mechanisms were existed such as carbonyl-scavenging. In conclusion, findings from the present study highlighted certain kinds of natural polyphenols as promising agents in counteracting RCS/lipid peroxidation-induced toxicity in amino acid, protein and cell models.