which has been found to be photosensitive. This compound is coupled to 

 a suitable coupliog component to produce a darlc colored azo dye at 

 sites of protein-bound tyrosine in tissues. A variant of this method, 

 which uses the production of an o,o'-dihydroxy azo dye from the coupling 

 of diazotyrosine to naphthoresorcinol, can be shown to couvert into 

 a deeply colored chelate complex upon the addition of suitable cations. 

 Comparison of the methods of diazotization-coupling with other similar 

 techniques appears to demonstrate for ail thèse methods a specificity 

 for protein bound tyrosine. The expérimental évidence leading to this 

 conclusion is presented. 



An évaluation of thèse techniques in regard to their sensitivity and 

 specificity reveals that interprétation of results must be predicated 

 upon the inhérent capacity of the reaction to solubilize spécifie proteins 

 and theknowledge that thèse **end-group" reactions may localize phenolic 

 or indolic compounds other than tyrosine and tryptophan. The conclusions 

 derived from the expérimental use of thèse reactions is evaluated. 



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