The crosslinking and antimicrobial properties of tunichrome

Tunichromes are small peptides containing one or more dehydrodopa derived units that have been identified in the blood cells of at least eleven species of tunicates. Incubation of tunichromes isolated from Ascidia nigra hemocytes (or model dopa-containing compounds) under oxidative conditions with either lysozyme, cytochrome c or ovalbumin resulted in a time-dependent polymerization of these test proteins to dimers, trimers, tetramers and potentially to other oligomers. These results indicate that the oxidation products of tunichromes possess inherent crosslinking properties. Hence it is possible that tunichromes participate in tunic production by forming adducts and crosslinks with structural proteins and/or carbohydrate polymers, similar to the well-understood process of insect cuticle hardening. Since such crosslinking potentials could also be beneficial for defense reactions against invading microorganisms, antibacterial activity of tunichromes was tested using both a radial diffusion assay and the Microtox(R) test. Tunichromes exhibited antimicrobial activity against gram-negative bacteria Escherichia coli and Photobacterium phosphorium. However, they did not show any antimicrobial activity against the gram-positive bacteria Staphylococcus aureus at the concentrations tested. We propose that the crosslinking and antimicrobial functions are both based on the reactivity of dehydrodopa units present in the tunichromes, and their subsequent ability to form highly reactive quinone methides.