Browning of Amino Acids and Proteins in Vitro: Insights Derived from an Electrophoretic Approach

An interaction of aldoses with aminogroups in aminoacids and proteins takes place "in vitro" under simulated physiological conditions (T=37°C, pH 7.4) which mimic the body environment. The order of reactivity was inversely correlated with the carbon number of the aldose molecule and generated yellow chromophores with characteristic fluorescent spectra. In the case of browning of aminoacids, highly microheterogeneous acidic and basic compounds were generated which were separated by electrophoresis in polyacrylamide gels where yellow pigments presented a different electrophoretic mobility than fluorescent homologs. Titration curves demonstrated a complex behaviour where in all cases pigmented compounds had a cathodic shift for pH5, while fluorescent homologs migrated to the anode. Browning of albumin by the same aldoses produced numerous microheterogeneus isospecies, with a more anionic pI than the original protein. Finally, highly glycosylated anionic albumin was detected in urines of normal human beings posing the possibility that browning of proteins affecting the isoelectric points of the protein may occour "in vivo".