Some studies of metal ions in protein systems

The fluorescence interaction of lanthanide ions with a number of amino acids, peptides and proteins has been investigated. Only Tb(III) fluorescence was enhanced by the substances tested. Tb(III) fluorescence was enhanced by tyrosine and peptides containing both tyrosine and tryptophan but not by tryptophan alone. The enhancement by tyrosine and the class A protein ribonuclease A was attributed to a collisional interaction, not binding at specific sites. Many class B proteins were shown to enhance Tb(III) fluorescence but the effect was not necessarily associated with vacant metal sites. A detailed study of the fluorescence interaction of Tb(III) with con A and porcine trypsin was made. None of the systems tested gave the huge Tb(III) fluorescence enhancement at small metal:protein ratio shown by transferrin and conalbumin. The interaction in these latter proteins involved rare earth binding at the Fe(III) sites. The interaction in the other class B proteins was probably due to a very complex interaction, possibly involving binding at a number of different sites and collisional interaction. [Continues.]