Tritium Labelling of Antibodies

A LARGE number of alkylating agents react with proteins while forming relatively stable compounds. Ethyl methane sulphonate (EMS) reacts in proteins mainly with non-ionized amino groups and ionized sulphydryl- and carboxyl-groups 1 . Below p H 8, the carboxyl groups constitute the majority of the alkylated groups, and above this p H-level the amino groups are also alkylated. By varying the p H it is possible to vary the pattern of the groups which are alkylated 2 , and this can be utilized to attach tritium-labelled groups to proteins. In the work recorded here we used tritium-labelled EMS, which was synthesized from commercially available tritiated ethanol (ethanol-2- T , Radiochemical Centre, Amersham) and methanesulphonic anhydride.