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 ε‐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.