Characterization and Biological Activities of Melanin from the Medicinal Fungi Ophiocordyceps sinensis

Pigments from pathogenic bacteria: a comprehensive update on recent advances

Bacterial pigments stand out as exceptional natural bioactive compounds with versatile functionalities. The pigments represent molecules from distinct chemical categories including terpenes, terpenoids, carotenoids, pyridine, pyrrole, indole, and phenazines, which are synthesized by diverse groups of bacteria. Their spectrum of physiological activities encompasses bioactive potentials that often confer fitness advantages to facilitate the survival of bacteria amid challenging environmental conditions. A large proportion of such pigments are produced by bacterial pathogens mostly as secondary metabolites. Their multifaceted properties augment potential applications in biomedical, food, pharmaceutical, textile, paint industries, bioremediation, and in biosensor development. Apart from possessing a less detrimental impact on health with environmentally beneficial attributes, tractable and scalable production strategies render bacterial pigments a sustainable option for novel biotechnological exploration for untapped discoveries. The review offers a comprehensive account of physiological role of pigments from bacterial pathogens, production strategies, and potential applications in various biomedical and biotechnological fields. Alongside, the prospect of combining bacterial pigment research with cutting-edge approaches like nanotechnology has been discussed to highlight future endeavours.

Production, physico-chemical properties and antioxidant activity of melanin from Annulohypoxylon stygium (Lév.) Y.M. Ju, J.D. Rogers and H.M. Hsieh

To obtain higher melanin production in liquid culture, culture conditions of Annulohypoxylon stygium (Lév.) Y.M. Ju, J.D. Rogers and H.M. Hsieh were optimised. The results showed that using single factor experiment and orthogonal test, the optimised production of melanin reached 2.20 g/L, which was 2.06 times higher than that of the control group. In addition, it was speculated that A. stygium melanin (AsM) was 3,4-dihydroxyphenylalanine (DOPA) melanin and showed an amorphous irregular structure. Moreover, it had good solubility in alkaline solution. AsM showed good antioxidant activity at a concentration of 500 mg/L, with DPPH, ABTS and OH radicals scavenging activities of 90.83%, 75.36% and 70.90%, respectively. AsM prevented alcohol-induced oxidative damage and oxidative stress in HepG2 cells by inhibiting the decrease of antioxidant key enzyme activity under alcohol stimulation. It was proved to have a great potential for application as a natural antioxidant and a substitute for synthetic pigments.

Date fruit melanin is primarily based on (-)-epicatechin proanthocyanidin oligomers

Plant-based melanin seems to be abundant, but it did not receive scientific attention despite its importance in plant biology and medicinal applications, e.g. photoprotection, radical scavenging, antimicrobial properties, etc. Date fruit melanin (DM) has complex, graphene-like, polymeric structure that needs characterization to understand its molecular properties and potential applications. This study provides the first investigation of the possible molecular composition of DM. High performance size-exclusion chromatography (HPSEC) suggested that DM contains oligomeric structures (569-3236 Da) and transmission electron microscopy (TEM) showed agglomeration of these structures in granules of low total porosity (10-1000 Å). Nuclear magnetic resonance (NMR) spectroscopy provided evidence for the presence of oligomeric proanthocyanidins and electron paramagnetic resonance (EPR) spectroscopy revealed a g-factor in the range 2.0034-2.005. Density functional theory (DFT) calculations suggested that the EPR signals can be associated with oligomeric proanthocyanidin structures having 4 and above molecular units of (-)-epicatechin. The discovery of edible melanin in date fruits and its characterization are expected to open a new area of research on its significance to nutritional and sensory characteristics of plant-based foods.

Fungi-derived natural antioxidants

In humans, exogenous antioxidants aid the endogenous antioxidant system to detoxify excess ROS generated during oxidative stress, thereby protecting the body against various diseases and stressful conditions. The majority of natural antioxidants available on the consumer market are plant-based; however, fungi are being recognized as alternative sources of various natural antioxidants such as polysaccharides, pigments, peptides, sterols, phenolics, alkaloids, and flavonoids. In addition, some exogenous antioxidants are exclusively found in fungi. Fungi-derived antioxidants exhibit scavenging activities against DPPH, ABTS, hydroxyl, superoxide, hydrogen peroxide, and nitric oxide radicals in vitro. Furthermore, in vivo models, application of fungal-derived antioxidants increase the level of various antioxidant enzymes, such as catalases, superoxide dismutases, and glutathione peroxidases, and reduce the level of malondialdehyde. Therefore, fungi-derived antioxidants have potential to be used in the food, cosmetic, and pharmaceutical industries. This review summarizes the antioxidant potential of different fungi (mushrooms, yeasts, and molds)-derived natural compounds such as polysaccharides, pigments, peptides, ergothioneine, ergosterol, phenolics, alkaloids, etc.