Intracellular metabolism of the apoproteins of pulmonary surfactant in rat lung.

We studied the metabolic turnover of the major apoproteins of pulmonary surfactant from rat lung and compared it with similar studies on the metabolism of dipalmitoyl phosphatidylcholine (DPPC). At varying times after the injection of tritiated leucine or tritiated palmitate we isolated the alveolar cells and alveolar fluid and measured the incorporation of the radioactive leucine into the 35,000-dalton (apoprotein A) and 10,000-dalton (apoprotein B) apoproteins of surfactant and of the radioactive palmitate into DPPC. Maximum labeling of apoprotein A in type II cells occurred within 1 h after injection of the precursor and declined in the next 19 h. The labeling of intracellular DPPC followed the same time course. The labeling of apoprotein B in type II cells was low, and the change of its labeling with time was not consistent with its being secreted by these cells. Both proteins were labeled in alveolar fluid and in alveolar macrophages. The results indicate that apoprotein A is synthesized by type II cells and is probably secreted as part of the surfactant complex. Apoprotein B is not secreted by type II cells with the same time course as apoprotein A or the lipids of surfactant, but our results do not define its origin or physiological purpose.