SPECTROPHOTOMETRIC EVIDENCE FOR INVOLVEMENT OF AROMATIC RESIDUES IN THE INTERACTION OF FERREASCIDIN WITH FERRIC ION

The iron(III)-binding properties of pyrocatechol and ferreascidin, a blood protein isolated from a stolidobranch ascidian (Pyura stolonifera), have been characterized by spectrophotometric titrations with metal ion in the presence of nitrilotriacetate (NTA) to maintain solubility under the conditions used (pH 7.0, I 0.2). Such studies of the model system have signified that the formation of the [Fe(NTA)(pyrocatechol)]2- complex is governed by a stability constant of 10(32.5) M-2. Corresponding studies of the interaction of iron(III) with the ascidian protein have provided spectrophotometric evidence for the coordination of each ferric ion to one tyrosine and two (3,4-dihydroxyphenyl)alanine (DOPA) residues, the interaction being characterized by an effective stability constant of 4 x 10(17) M-1 under the conditions studied (pH 7.0, I 0.2). Finally, a case is made for potential biological relevance of the iron(III)-ferreascidin interaction in the curing of this DOPA protein for the repair of wounds and/or securement of the ascidian to its substratum.