Radiolabeling of the hydrophobic components of lung surfactant with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine

Studies of the metabolism and distribution of lung surfactant are aided by use of radiolabeled surfactant or surfactant components. These studies have often made use of [3H]- or [14C]phosphatidylcholine. Analysis of the lung content of surfactant containing these β-emitting labels usually requires tissue digestion, use of scintillation fluids, and significant correction for quenching of photon production. Because use of a γ-emitting isotope would obviate these requirements, we have investigated the use of 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID), a lipophilic photoactivatable compound, to radiolabel pulmonary surfactant. Our results indicate that, during photoactivation, products of [125I]TID are produced that result in radiolabeling of both the lipid and protein components of extracted porcine surfactant. Separation of radiolabeled surfactant from hydrophobic nonlabeling photolysis products was accomplished by gel chromatography. Exposure of surfactant (34 μmol/ml) to [125I]TID under labeling conditions resulted in incorporation of 45.3 ± 5.1% of the radiolabel. Incorporation of radiolabel in the various phospholipids of lung surfactant was approximately equivalent. Lipophilic surfactant apoproteins were also radiolabeled. Finally, both in vitro and in vivo testing of radiolabeled surfactant (0.1 μCi/mg) revealed full retention of surface tension lowering ability.