Radioiodination of transforming growth factor-beta (TGF-beta) in a modified Bolton-Hunter reaction system

We have optimized the use of the Bolton-Hunter reagent to prepare 125I-labeled transforming growth factor-beta (TGF-beta). Conditions were developed to obtain monovalent modification of human-recombinant TGF-beta 2 (hrTGF-beta 2) at a basic pH necessary for efficient protein acylation (> or = 26% of theoretical) while obviating the problems of TGF-beta aggregation/precipitation. The purified Bolton-Hunter labeled hrTGF-beta 2 had a specific activity of 1.8-2.1 microCi/pmol, and the 125I label was fully acid-precipitable. [125I]hrTGF-beta 2 was electrophoretically indistinguishable from unlabeled starting material and displayed full immunoreactivity with polyclonal anti-TGF-beta 2 antibody. Both hrTGF-beta 2 and Bolton-Hunter-labeled [125I]hrTGF-beta 2 inhibited the growth of mink lung epithelial cells with equal efficacy. These data validate a modified conjugation-iodination method for TGF-beta and invite general use of the Bolton-Hunter reagent for iodination of other TGF-beta isoforms and peptides similarly susceptible to precipitation/aggregation under standard Bolton-Hunter incubation conditions.