Production and properties of non-cytotoxic pyomelanin by laccase and comparison to bacterial and synthetic pigments

Pyomelanin is a polymer of homogentisic acid synthesized by microorganisms. This work aimed to develop a production process and evaluate the quality of the pigment. Three procedures have been elaborated and optimized, (1) an HGA-Mn(2+) chemical autoxidation (Pyo(CHEM) yield 0.317 g/g substrate), (2) an induced bacterial culture of Halomonas titanicae through the 4-hydroxyphenylacetic acid-1-hydroxylase route (Pyo(BACT), 0.55 g/L), and (3) a process using a recombinant laccase extract with the highest level produced (Pyo(ENZ), 1.25 g/g substrate) and all the criteria for a large-scale prototype. The chemical structures had been investigated by (13)C solid-state NMR (CP-MAS) and FTIR. C(ar)–C(ar) bindings predominated in the three polymers, C(ar)–O–C(ar) (ether) linkages being absent, proposing mainly C(3)-C(6) (α-bindings) and C(4)-C(6) (β-bindings) configurations. This work highlighted a biological decarboxylation by the laccase or bacterial oxidase(s), leading to the partly formation of gentisyl alcohol and gentisaldehyde that are integral parts of the polymer. By comparison, Pyo(ENZ) exhibited an M(w) of 5,400 Da, was hyperthermostable, non-cytotoxic even after irradiation, scavenged ROS induced by keratinocytes, and had a highly DPPH-antioxidant and Fe(3+)-reducing activity. As a representative pigment of living cells and an available standard, Pyo(ENZ) might also be useful for applications in extreme conditions and skin protection.