An enzymatic approach to the labeling of tryptophan residues in peptides and proteins

One of the main obstacles to the analysis of peptide action at the cellular level is the design of radioactive labeling procedures which do not significantly alter the biological activity of the labeled peptide. Of the two isotopes commonly used to obtain compounds of high specific radioactivity (3H, 1251), only tritium labeling produces true tracers without modification of physical or chemical properties. However, since suitable unsaturated or halogenated precursors for catalytic reduction with tritium gas can only be obtained in a few exceptional cases, iodination is the method commonly employed. The resulting difficulties associated with the production of an analog have been well documented for insulin and glucagon [l-5] , and even more clearly for angiotensin and posterior pituitary hormones [6-91. The recent discovery and isolation of a novel bacterial hemoprotein enzyme, named indolyl3alkane cr-hydroxylase [IO] , suggested an alternative and partially enzymatic approach to labeling of proteins and peptides. The enzyme catalyzes the formation of an unsaturated intermediate (3.alkylidene indoline) which can be isolated if both the carboxy and a-amino group of tryptophan are protected by substitution [ 1 I] . In this paper we present a summary of new obser-