In vivo incorporation of 14C-phenylalanine into ascidian tunichrome

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

Plicatamide: A lead to the biosynthetic origins of the tunichromes?

Plicatamide is a modified octapeptide from the ascidian Styela plicata having the structure Phe-Phe-His-Leu-His-Phe-His-decarboxyDeltaDOPA (where decarboxyDeltaDOPA = decarboxy-(E)-alpha,beta-dehydro-3, 4-dihydroxyphenylalanine). Edman sequencing, tandem mass spectrometry, and proton NMR have been used to characterize 100 microg of the compound. Plicatamide represents an important link between two classes of biomolecules: the tunichromes which share an oxidatively decarboxylated C-terminus and higher molecular weight DOPA-polypeptides. The 8-residue sequence provides the first opportunity to investigate the biosynthetic origins of the tunichrome family by molecular biological techniques.