Am. J. Respir. Cell Mol. Biol., Vol 11, No. 2, Aug 1994, 240-248.
Synthesis of type II cell lamellar body lysozyme-15 kD protein (lbl-15) by perfused rat lung
MF Beers, CY Kim, C Dodia and AB Fisher
Institute for Environmental Medicine University of Pennsylvania School of Medicine, Philadelphia 19104-6068.
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 35S 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 35S-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 35S-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.(ABSTRACT
TRUNCATED AT 250 WORDS)
