Oxidative transformation of a tunichrome model compound provides new insight into the crosslinking and defense reaction of tunichromes

Tunichromes, small oligopeptides with dehydrodopa units isolated from the blood cells of ascidians, have been implicated in the defense reactions, metal binding, wound repair, or tunic formation. Their instability and high reactivity has severely hampered the assessment of their biological role. Experiments conducted with the model compound, 1,2-dehydro-N-acetyldopamine, indicated that the instability of tunichromes is due to this basic structure. Exposure of this catecholamine derivative to even mild alkaline condition such as pH 7.5 causes rapid nonenzymatic oxidation. High performance liquid chromatography and mass spectrometry studies confirmed the production of dimeric and other oligomeric products in the reaction mixture. The nonenzymatic reaction seemed to proceed through the intermediary formation of semiquinone free radical and superoxide anion. Ultraviolet and visible spectral studies associated with the oxidation of tunichromes isolated from Ascidia nigra by tyrosinase indicated the probable formation of oligomeric tunichrome products. Attempts to monitor the polymerization reaction by mass spectrometry ended in vain. Tunichrome also exhibited instability in mild alkaline conditions generating superoxide anions. Based on these studies, a possible role for oxidative transformation of tunichrome in defense reaction, tunic formation and wound healing is proposed. (C) 2017 Elsevier Inc. All rights reserved.