Characteristics of amino acid metabolism by isolated alveolar type II cells

Alveolar type II cells of the lung are important in producing the lipoprotein surfactant. Most studies about metabolism in type II cells have focussed on lipid precursors for phospholipid metabolism. Surfactant contains a unique apoprotein; yet relatively little is known about the metabolism of amino acids by type II cells. Type II cells were isolated using density gradient centrifugation followed by centrifugal elutriation. Alanine (Ala), leucine (Leu), valine (Val), and phenylalanine (Phe) incorporation into protein and lipid and oxidation to CO2 was measured after the cells were incubated for 2 h. For alanine metabolism, 22% of total radioactivity from alanine was incorporated into protein, 20% into lipid, and 58% oxidized to CO2. For leucine, 51% was incorporated into protein, 23% into lipid, and 22% oxidized to CO2. Fifty percent of radioactivity from valine metabolism was incorporated into protein, 5% into lipid, and 47% oxidized to CO2. Virtually all (95%) of phenylalanine, however, was utilized for protein synthesis only. Puromycin and cycloheximide decreased protein synthesis from Ala, Leu, and Phe but had little affect on Ala and Leu metabolism to lipid or CO2. The hypolipidemic drug clofibrate inhibited all aspects of amino acid metabolism. In summary, type II cell amino acid metabolism is regulated similar to that of cells from skeletal muscle and adipose tissue, but in contrast to hepatocytes, type II cells readily oxidize valine and utilize leucine for lipid as well as protein synthesis.