Department of Plant Sciences, School of Life Sciences, University of Hyderabad, Hyderabad, 500046, India.
Department of Botany, Bharathidasan Government College for Women, Puducherry, U.T. - 605003, India.
Sci Rep. 2020 Jun 2;10(1):8925. doi: 10.1038/s41598-020-65803-6.
Melanins are chemically diverse ubiquitous pigments found across the life forms synthesized via different biochemical pathways mainly from L-tyrosine or acetyl CoA. Though few reports suggest the possibility of tryptophan-based melanin synthesis, however, such tryptophan-based melanin and its biosynthesis remained a biochemical riddle. Here we report tryptophan-based melanin production by bacterium, Rubrivivax benzoatilyticus JA2. Aerobic cultures of strain JA2 produced brown pigment when grown on L-tryptophan-containing media. Purified pigment showed typical physico-chemical properties of melanin. Further, extensive spectroscopic studies revealed that pigment is an amorphous, indole-type polymer with stable free radical centers. Further, hydrolysis of the brown pigment revealed the presence of indole moiety, confirming the indolic nature of the pigment. Demonstration of in vitro and in vivo pigment synthesis directly from L-tryptophan or hydroxytryptophan confirms tryptophan-based melanin synthesis in strain JA2. Interestingly, canonical melanin biosynthetic inhibitors did not affect the pigment synthesis indicating possible non-canonical tryptophan-based melanin biosynthesis in strain JA2. Further, the exometabolite profiling and precursor feeding studies suggests that L-tryptophan converted to hydroxytryptophan/hydroxyindoles and their subsequent polymerization lead to the formation of melanin. The current study sheds light on biosynthetic diversity of melanins and L-tryptophan can be a potential precursor for melanin synthesis in life forms.
黑色素是化学性质多样的普遍存在的色素，存在于各种生命形式中，主要通过不同的生化途径合成，来自 L-酪氨酸或乙酰辅酶 A。虽然有少数报道表明色氨酸基黑色素合成的可能性，但这种色氨酸基黑色素及其生物合成仍然是一个生化谜题。在这里，我们报告了细菌 Rubrivivax benzoatilyticus JA2 产生色氨酸基黑色素。当在含有 L-色氨酸的培养基上生长时，JA2 菌株的需氧培养物会产生棕色色素。纯化的色素表现出黑色素的典型物理化学性质。此外，广泛的光谱研究表明，该色素是一种无定形的吲哚型聚合物，具有稳定的自由基中心。此外，棕色色素的水解揭示了吲哚部分的存在，证实了色素的吲哚性质。体外和体内色素直接从 L-色氨酸或羟色氨酸合成的证明证实了 JA2 菌株中色氨酸基黑色素的合成。有趣的是，典型的黑色素生物合成抑制剂不影响色素合成，表明 JA2 菌株中可能存在非典型的色氨酸基黑色素生物合成。此外，外代谢产物分析和前体喂养研究表明，L-色氨酸转化为羟色氨酸/羟基吲哚，随后聚合导致黑色素的形成。本研究揭示了黑色素生物合成的多样性，L-色氨酸可以作为生命形式中黑色素合成的潜在前体。
