Characterization of rabbit lung lysosomes and their role in surfactant dipalmitoylphosphatidylcholine catabolism

Although alveolar surfactant is rapidly catabolized in adult rabbit lungs, the pathways have not been characterized. Pathways of surfactant secretion and recycling involve lamellar bodies and multivesicular bodies, organelles shown to be related to lysosomes by cytochemistry and autoradiography. Since lysosomes are central to intracellular catabolic events, it is possible that lysosomes are involved in intrapulmonary surfactant catabolism. Lysosomes relatively free of contaminating organelles (as determined morphologically and by marker enzymes for mitochondria, endoplasmic reticulum, peroxisomes, and plasma membranes) were obtained from post-lavage lung homogenates of 1-kg rabbits by differential centrifugation in buffered sucrose and gradient separation in percoll (density, 1.075-1.165). The role of lung lysosomes in catabolism of dipalmitoylphosphatidylcholine (DPC) was then studied in rabbits killed 4, 12, and 24 h following intratracheal injection of [3H]DPC and [14C] dihexadecyl phosphatidylcholine (DPC-ether). While equal amounts of label were in the lamellar body containing fractions at 4 h, nearly 6-fold more DPC-ether label than DPC label was recovered in the lysosomal fractions. By 24 h, there was 15-fold more DPC-ether in the lysosomes. This is the first report of successful isolation of lysosomes relatively free of other organelles from rabbit lungs. The tracer studies indicate DPC and DPC-ether follow similar intracellular processing after alveolar uptake. The subsequent accumulation of the ether analog in the lysosomal fractions supports a role for these organelles in surfactant DPC catabolism.