Phyllobilins as a challenging diverse natural product class: Exploration of pharmacological activities

Novel Types of Phyllobilins in a Fern - Molecular Reporters of the Evolution of Chlorophyll Breakdown in the Paleozoic Era

Breakdown of chlorophyll (Chl), as studied in angiosperms, follows the pheophorbide a oxygenase/phyllobilin (PaO/PB) pathway, furnishing linear tetrapyrroles, named phyllobilins (PBs). In an investigation with fern leaves we have discovered iso-phyllobilanones (iPBs) with an intriguingly rearranged and oxidized carbon skeleton. We report here a key second group of iPBs from the fern and on their structure analysis. Previously, these additional Chl-catabolites escaped their characterization, since they exist in aqueous media as mixtures of equilibrating isomers. However, their chemical dehydration furnished stable iPB-derivatives that allowed the delineation of the enigmatic structures and chemistry of the original natural catabolites. The structures of all fern-iPBs reflect the early core steps of a PaO/PB-type pathway and the PB-to-iPB carbon skeleton rearrangement. A striking further degradative chemical ring-cleavage was observed, proposed to consume singlet molecular oxygen (1O2). Hence, Chl-catabolites may play a novel active role in detoxifying cellular 1O2. The critical deviations from the PaO/PB pathway, found in the fern, reflect evolutionary developments of Chl-breakdown in the green plants in the Paleozoic era.

Structural Characterization, and Antioxidative and Anti-inflammatory Activities of Phylloxanthobilins in Tropaeolum majus, a Plant with Relevance in Phytomedicine

Tropaeolum majus (garden nasturtium) is a plant with relevance in phytomedicine, appreciated not only for its pharmaceutical activities, but also for its beautiful leaves and flowers. Here, we investigated the phytochemical composition of senescent nasturtium leaves. Indeed, we identified yellow chlorophyll catabolites, also termed phylloxanthobilins, which we show to contribute to the bright yellow color of the leaves in the autumn season. Moreover, we isolated and characterized the phylloxanthobilins from T. majus, and report the identification of a pyro-phylloxanthobilin, so far only accessible by chemical synthesis. We show that the phylloxanthobilins contribute to bioactivities of T. majus by displaying strong anti-oxidative effects in vitro and in cellulo, and anti-inflammatory effects as assessed by COX-1 and COX-2 enzyme inhibition, similar to other bioactive ingredients of T. majus, isoquercitrin, and chlorogenic acid. Hence, phylloxanthobilins could play a role in the efficacy of T. majus in the treatment of urinary tract infections, an established indication of T. majus. With the results shown in this study, we aid in the completion of the phytochemical profile of T. majus by identifying additional bioactive natural products as relevant components of this medicinal plant.

Recent advances in anti-inflammatory active components and action mechanisms of natural medicines

Inflammation is a series of reactions caused by the body's resistance to external biological stimuli. Inflammation affects the occurrence and development of many diseases. Anti-inflammatory drugs have been used widely to treat inflammatory diseases, but long-term use can cause toxic side-effects and affect human functions. As immunomodulators with long-term conditioning effects and no drug residues, natural products are being investigated increasingly for the treatment of inflammatory diseases. In this review, we focus on the inflammatory process and cellular mechanisms in the development of diseases such as inflammatory bowel disease, atherosclerosis, and coronavirus disease-2019. Also, we focus on three signaling pathways (Nuclear factor-kappa B, p38 mitogen-activated protein kinase, Janus kinase/signal transducer and activator of transcription-3) to explain the anti-inflammatory effect of natural products. In addition, we also classified common natural products based on secondary metabolites and explained the association between current bidirectional prediction progress of natural product targets and inflammatory diseases.

New Perspective for Macroalgae-Based Animal Feeding in the Context of Challenging Sustainable Food Production

Food production is facing challenging times due to the pandemic, and climate change. With production expected to double by 2050, there is a need for a new paradigm in sustainable animal feed supply. Seaweeds offer a highly valuable opportunity in this regard. Seaweeds are classified into three categories: brown (Phaeophyceae), red (Rhodophyceae), and green (Chlorophyceae). While they have traditionally been used in aquafeed, their demand in the feed market is growing, parallelly increasing according to the food demand. Additionally, seaweeds are being promoted for their nutritional benefits, which contribute to the health, growth, and performance of animals intended for human consumption. Moreover, seaweeds contain biologically active compounds such as polyunsaturated fatty acids, antioxidants (polyphenols), and pigments (chlorophylls and carotenoids), which possess beneficial properties, including antibacterial, antifungal, antiviral, antioxidant, and anti-inflammatory effects and act as prebiotics. This review offers a new perspective on the valorization of macroalgae biomass due to their nutritional profile and bioactive components, which have the potential to play a crucial role in animal growth and making possible new sources of healthy food ingredients.

Phyllobilins - Bioactive Natural Products Derived from Chlorophyll - Plant Origins, Structures, Absorption Spectra, and Biomedical Properties

Phyllobilins are open-chain products of the biological degradation of chlorophyll a in higher plants. Recent studies reveal that phyllobilins exert anti-oxidative and anti-inflammatory properties, as well as activities against cancer cells, that contribute to the human health benefits of numerous plants. In general, phyllobilins have been overlooked in phytochemical analyses, and - more importantly - in the analyses of medicinal plant extracts. Nevertheless, over the past three decades, > 70 phyllobilins have been identified upon examination of more than 30 plant species. Eight distinct chromophoric classes of phyllobilins are known: phyllolumibilins (PluBs), phylloleucobilins (PleBs), phylloxanthobilins (PxBs), and phylloroseobilins (PrBs)-each in type-I or type-II groups. Here, we present a database of absorption and fluorescence spectra that has been compiled of 73 phyllobilins to facilitate identification in phytochemical analyses. The spectra are provided in digital form and can be viewed and downloaded at www.photochemcad.com. The present review describes the plant origin, molecular structure, and absorption and fluorescence features of the 73 phyllobilins, along with an overview of key medicinal properties. The review should provide an enabling tool for the community for the straightforward identification of phyllobilins in plant extracts, and the foundation for deeper understanding of these ubiquitous but underexamined plant-derived micronutrients for human health.

The advent of phyllobilins as bioactive phytochemicals – natural compounds derived from chlorophyll in medicinal plants and food with immunomodulatory activities

The degradation of the green plant pigment chlorophyll has fascinated chemists and biologists alike over the last few decades. Bioactivities of the compounds formed in this biochemical degradation pathway, however, have only come to light recently. These natural compounds that are formed from chlorophyll during plant senescence are now called phyllobilins. In this review, we shortly discuss chlorophyll degradation and outline the so-far known bioactivities of selected phyllobilins (phylloleucobilin, dioxobilin-type phylloleucobilin, and phylloxanthobilin), and we also highlight the recently discovered immunomodulatory effects of a yellow phylloxanthobilin.

Side-chain modifications of phyllobilins may not be essential for chlorophyll degradation in Arabidopsis

Disposing efficiently and safely chlorophyll derivatives during senescence requires a coordinated pathway that is well conserved throughout green plants. The PAO/phyllobilin pathway catalyzes the degradation of the chlorophyll during senescence and allows detoxification of the pigment and its subsequent export from the chloroplast. Although most of the chloroplastic reactions involved in chlorophyll degradation are well understood, the diversity of enzymes responsible for downstream modifications of non-phototoxic phyllobilins remains to be explored. More than 40 phyllobilins have been described to date, but only three enzymes catalyzing side-chain reactions have been identified in Arabidopsis thaliana, namely, TIC55, CYP89A9, and MES16. Here, by generating a triple mutant, we evaluate the extent to which these enzymes are influencing the rate and amplitude of chlorophyll degradation at the metabolite as well as its regulation at the transcriptome level. Our data show that major side-chain modifications of phyllobilins do not influence significantly chlorophyll degradation or leaf senescence, letting the physiological relevance of their striking diversity an open question.

Facile retro-Dieckmann cleavage of a pink phyllobilin: new type of potential downstream steps of natural chlorophyll breakdown

In senescent leaves of higher plants, colourless chlorophyll (Chl) catabolites typically accumulate temporarily, and undergo natural oxidation, in part, to yellow- and pink-coloured phyllobilins (PBs). The latter, also classified as phylloroseobilins (PrBs), represent the final currently established products of Chl-breakdown, possibly playing important roles in metabolism. However, PrBs, themselves, do not accumulate in the leaves. Indeed, the original PrB identified, then classified as a pink Chl-catabolite (PiCC), is remarkably instable in methanolic solution. As reported here, PiCC readily converts at room temperature into yellow tetrapyrroles. The deduced main process, a retro-Dieckmann reaction, cleaves open its ring E moiety, the α-methoxycarbonyl-cyclopentanone unit characteristic of the Chls and of the natural Chl-derived PBs. This readily occurring reaction of the PiCC represents an unprecedented skeletal transformation of a PB, furnishing a cross-conjugated biladiene with a basic structure more similar to the heme-derived bilins.

Graphical abstract

Tetrapyrrolic Pigments from Heme- and Chlorophyll Breakdown are Actin-Targeting Compounds

Chlorophyll and heme are among the "pigments of life", tetrapyrrolic structures, without which life on Earth would not be possible. Their catabolites, the phyllobilins and the bilins, respectively, share not only structural features, but also a similar story: Long considered waste products of detoxification processes, important bioactivities for both classes have now been demonstrated. For phyllobilins, however, research on physiological roles is sparse. Here, we introduce actin, the major component of the cytoskeleton, as the first discovered target of phyllobilins and as a novel target of bilins. We demonstrate the inhibition of actin dynamics in vitro and effects on actin and related processes in cancer cells. A direct interaction with G-actin is shown by in silico studies and confirmed by affinity chromatography. Our findings open a new chapter in bioactivities of tetrapyrroles-especially phyllobilins-for which they form the basis for broad implications in plant science, ecology, and physiology.