Ion beam tritium labeling of proteins and peptides

A general method for tritiating proteins, peptides, and other nonvolatile organic compounds has been developed. A carefully controlled particle beam composed of T3+ and T2+ ions and fast T2 molecules is accelerated into a sample target within a vacuum chamber. This beam method has been used to tritiate ribonuclease A, porcine pancreatic elastase, thermolysin, soybean trypsin inhibitor, alpha 1-protease inhibitor, and the peptide aldehydes leupeptin and antipain. After removal of all readily exchangeable tritium, the products were obtained in 32-83% yields with specific radioactivities of 18-856 Ci/mol. The products were carefully characterized, shown to be chemically pure, and to have complete biological activity. Simple tritium hydrogen exchange accounts for at least 82% of the reaction pathway with proteins and for 100% of the reaction with the peptide aldehydes. The ion beam method is a mild procedure for general tritium labeling of fragile protein macromolecules and other sensitive biological molecules.