SYNTHESIS OF TYPE-II CELL LAMELLAR BODY LYSOZYME 15-KD PROTEIN (LBL-15) BY PERFUSED RAT LUNG

Isolated alveolar type II pneumocytes of the rat have been shown to secrete a 14 to 15 kD protein that has some sequence homology and immunoreactivity with lysozyme. Using immunochemical analyses of rat lung subcellular fractions and S-35 metabolic labeling of isolated perfused lung preparations, we studied the subcellular distribution and synthetic pathway for this protein. SDS-PAGE and Western blotting of lamellar bodies (LB) using a polyclonal anti-human lysozyme (anti-HLZ) demonstrated a single band at 15 kD that was significantly enriched over rat lung homogenates, isolated lysosomes, and type II cell lysates. This 15 kD protein isolated from LB by immunoprecipitation with anti-HLZ also demonstrated functional lysozyme activity and was termed lamellar body lysozyme (lbl-15). Analysis of LB and surfactant (SF) isolated from 12 separate perfused lung preparations labeled for 6 h with S-35-labeled amino acids demonstrated that lbl-15 represented a significant portion of the radiolabeled LB proteins (5.9% of total LB radioactivity). Lamellar bodies and an extracellular fraction (surfactant) obtained from rat lungs perfused with S-35-methionine-cysteine for varying times both showed time-dependent appearance of lbl-15. At all time points, the specific activity of lbl-15 was greater in LB than in SF. The kinetics for appearance of lbl-15 in LB and SF was similar to that for surfactant protein A. These results indicate that in the rat lung, type II cells synthesize a 15 kD protein (lbl-15) that is secreted into the alveolar space via an organellar pathway involving LB. This protein appears identical to previously described proteins with sequence homology, immunologic cross reactivity, and functional identity to rat lysozyme.