Antimicrobial binding of a radiolabeled cationic neutrophil granule protein

A 57,000-dalton protein (CAP57) purified from human polymorphonuclear leukocytes has antimicrobial activity against a number of gram-negative bacteria. We developed a procedure using solid-phase Iodo-gen to radiolabel CAP57 without destroying its antibacterial activity. Iodinated and native CAP57 were electrophoretically identical. Autoradiographs of sodium dodecyl sulfate-polyacrylamide gels revealed greater than 95% of the 125I in a single heavy band in the 57,000-molecular-weight region. The quantity of [125I]CAP57 bound to bacterial test strains was directly proportional to the sensitivity to CAP57.