Redox chemistry of bioactive plant secondary metabolites

Secondary metabolites (SM) occur in relatively huge diversity and amounts in plants. The general opinion is that their main benefit is to contribute to plant chemical defence against microbial pathogens and herbivore predators in which they cause toxic effects, but also against competing plants (allelopathy). Conversely, plants SM also are renowned for their beneficial effects in medical applications when they are administered in low dosages. In both cases, binding to receptor pockets of proteins, such as cholinergic, adrenergic, serotonergic, GABAergic, for adenosine and glutamate, or interacting with secondary messengers (e.g., cAMP, cGMP) or hormonal regulation (e.g., auxins, insect juvenile hormones) are suggested to represent a major component of their mode of action. SM also have been shown to participate in redox chemical reactions, in which their reducing power can scavenge reactive oxygen species (ROS) that are induced by abiotic and biotic stress. This antioxidative effect is especially appreciated if SM are to serve as food additives that are aimed at improving consumer health by slowing the onset of degenerative diseases, such as cancer and others. These beneficial effects, however, may be changed into detrimental if, instead of ROS, molecular oxygen is reduced. As a consequence, oxidative stress is triggered. These redox chemical reactions are inevitable in the cytoplasm of cells in living organisms and thus can be regarded as a component of pre-receptor chemistry. This project explores this pre-receptor redox chemistry of a selection of about 70 plant SM based on their documented biological activity, mostly toxic effects against microbes and insects, but also on competing plants (allelopathy), and their affiliation to the most prominent SM classes. The main hypothesis of the project purports that redox chemical reactions represent a non-negligible component of SM biological activity. To explore this, various chemical, electrochemical and mass spectrometric methods will be employed. Redox chemical reactions with ROS will be explored by specifically modified systems and variants of the deoxyribose degradation assay. Specifically, these assays explore if the test compound can reduce molecular oxygen, superoxide anion radical, hydrogen peroxide, or hydroxyl radicals, and the catalyst iron (in complex with the test compound or EDTA, a chelator used widely in experiments). Voltammetry, differential-pulse and square-wave, informs about reversibility and irreversibility of test compound autoxidation. The reactions following autoxidation of the test compound are investigated by high resolution mass spectroscopy.

Secondary metabolites occur in huge structural diversity. Generally, they are assumed to help an organism to survive in a highly competitive environment by protecting it against predator attacks or suppression of rival organisms. Amongst others, secondary metabolites may react with reactive oxygen species (ROS) that are induced by abiotic or biotic stress. The redox reactions of the bioactive secondary metabolites can form an important part of their prereceptor chemistry. The goal of the project was the investigation of this specific redox chemistry on selected examples. The redox properties and interactions were explored by chemical methods such as the deoxyribose degradation assay and by physico-chemical methods, such as high resolution mass spectrometry, and electrochemical methods, differential pulse and square-wave voltammetry. Within the project, the research interest was focused on substances with assumed innovative perspectives in plant or human physiology. The results yielded a complicated categorization of anti- or pro-oxidants. The substances classified as toxins by their pro-oxidant activities such as quinolinic or 3hydroxyanthranilic acid may behave also as antioxidants with possible protective activities on cells under oxidative stress. On the contrary, substances assumed to be protective against some effects of oxidative stress, e.g. flavonoids, phenolic acids, and others, can increase the levels of ROS leading to their cytotoxicity. The project results demonstrated that the metabolite capability of oxidative decreasing and interactions with ROS strongly depends on the present chemical milieu and, generally, question the general classifications as cytoprotective agents and cytotoxins.