Synthesis of sulfur-35 reagents for protein labeling

Two 35 S reagents were developed to radiolabel proteins. The first reagent, a N-hydroxysuccinimide (NHS) ester (SMSB), acylates the e-amino group of lysine residues in proteins. The second reagent, an aldehyde (MSAPPA), labels lysine residues via reductive alkylation. Comparing the two methods, the reductive alkylation method labeled proteins over a broader pH range with higher overall radiochemical yield. The biological activity of the proteins did not change after labeling with these 35 S reagents.