125I-glycoconjugate labels for identifying sites of protein catabolism in vivo: effect of structure and chemistry of coupling to protein on label entrapment in cells after protein degradation

Residualizing radioactive labels are designed to remain entrapped within cells following degradation of a carrier protein, and have been used for identification of the tissue and cellular sites of plasma protein catabolism. In this study the authors describe a convenient synthesis and purification of a series of 125I-labeled glycoconjugates, and an evaluation of their efficiency of retention in liver following degradation of a model carrier protein, asialofetuin. Glycoconjugates were prepared in 65-90% yield by reductive amination of reducing sugars with aromatic amines using NaBH3CN. The products were purified in a single ion-exchange chromatographic step, and then labeled with 125I. The derivatives prepared were mono-and disubstituted lactitol-,cellobiitol-and glucitol-[125I]tyramine and lactitol-[125I]tyrosine. 125I-Glycoconjugates were coupled to asialofetuin using either cyanuric chloride or, for lactose-containing labels, by treatment with galactose oxidase followed by reductive amination with NaBH3CN. The authors observed that degradation products from larger, disubstituted glycoconjugates were retained more efficiently than those from smaller and monosubstituted derivatives, and that glycoconjugates coupled to protein via reductive amination were retained in the body more efficiently than those coupled by cyanuric chloride. Overall, dilactitol-[125I]tyramine coupled to protein by reductive amination was entrapped most efficiently in liver