Colorless chlorophyll catabolites

Over the last sixteen years only, were colorless tetrapyrroles discovered as chlorophyll catabolites in senescent leaves and found to accumulate there as the "final" tetrapyrrolic breakdown products of chlorophyll. Very recently, we were able to identify such natural breakdown products of chlorophyll in apples also and to characterize their chemical structures. Our recent findings lead us to propose that colorless chlorophyll catabolites represent previously overlooked components of human nutrition. While one of these catabolites has been shown to be an effective antioxidant, their possibly physiologically relevant other properties still are largely unexplored. A first detailed study of such properties of these catabolites is planned, which may be of specific interest for components of the nutrition, and which could be physiologically important in humans and animals. In the present project a search for such chlorophyll catabolites in vegetables and ripe fruit is planned and systematic studies of the availability of non-green tetrapyrrolic chlorophyll catabolites in senescent plant material. The newly gained knowledge is to be used to obtain multi-gram samples of those catabolites, which we have identified as components of the typical human nutrition, in order to submit them to exploratory pharmacological examinations. The research is geared at obtaining information on the presence of non-green chlorophyll catabolites as components of typical plant-derived human nutrition, on basic and physiologically interesting chemical properties of such chlorophyll catabolites, and on their ingestion from food. This work should also yield a first knowledge base, to help assess possible physiological effects of such catabolites as natural food components and to prepare for their possible further exploration as natural food components, which may also be of interest for pharmaceutical and medical applications.

Chlorophyll is the key pigment of plants. The seasonal signs of chlorophyll breakdown - fall colours in the foliage of deciduous plants, and typical colour changes in ripening fruit - are most fascinating, and probably also the most visible, signs of life on earth. Many important basic facts on chlorophyll breakdown, which meanwhile have become textbook knowledge, have remained elusive until around the turn of this century. To this subject, our group has made major contributions. The present project explored the occurrence of colourless chlorophyll catabolites in natural sources and their availability in them for the purpose of the efficient preparative isolation of such still hardly known types of natural products. Indeed, fall leaves of a variety of deciduous trees proved to be efficient sources of non-fluorescent chlorophyll catabolites, colourless representatives of the formyloxobilin-type chlorophyll catabolites, or type-I phyllobilins. However, some leaves, instead, were found to contain dioxobilin-type catabolites, or type-II phyllobilins. The critical catabolic transition from type-I to type-II phyllobilins was also identified as an enzyme-catalized removal of a formyl group. Thus, chlorophyll breakdown in higher plants was revealed to be branching out to different types of bilins, relatives of heme degradation products, such as bilirubin. Relevant chemical properties of several colourless chlorophyll catabolites as naturally occurring antioxidants were investigated, as well as endogenous manifestations of their activities as efficient antioxidants. An unexpected endogenous oxidative transformation was discovered, which provided a natural path from colourless to yellow chlorophyll catabolites, which are known to contribute to the fall colours in leaves of deciduous trees and in other senescent leaves. Our work has opened the preparative access to important nonfluorescent chlorophyll catabolites as natural products. At the same time, it strengthened the view that tetrapyrrolic catabolites of chlorophyll are not mere detoxification products, but that they may play still unknown physiological roles. The presence of colourless chlorophyll catabolites in plant-based parts of our nutrition made us also interested in their physiological effect in mammals. In exploratory experiments laboratory animals were treated with a colourless nonfluorescent chlorophyll catabolite and the metabolic fate of these was studied. A rapid endogenous hydrolytic partial degradation was identified.