IDENTIFICATION OF A PUTATIVE SURFACTANT CONVERTASE IN RAT LUNG AS A SECRETED SERINE CARBOXYLESTERASE

In the alveolar lumen, pulmonary surfactant converts from the contents of secreted lamellar bodies to tubular myelin to apoprotein-depleted vesicles during respiration. Using an in vitro system, researchers hav e reported that the conversion of tubular myelin to vesicles is blocke d by inhibitors of serine hydrolase activity and have tentatively ascr ibed ''convertase'' activity to a diisopropyl fluorophosphate (DFP)-bi nding protein in mouse bronchoalveolar lavage (BAL). We purified and s equenced the homologous enzyme from rat BAL fluid. Amino acid sequence from the amino terminus and an internal cyanogen bromide peptide of t he purified rat DFP-binding protein perfectly match the sequence of th e carboxylesterase ES-2. Although ES-2 was initially cloned from liver , we found a 1.8-kilobase mRNA for ES-2 in decreasing relative abundan ce in rat liver, kidney, and lung but not in heart or spleen. Although further studies are required to establish the identity between ''conv ertase'' and ES-2 or a homologous member of the carboxylesterase famil y, our results raise the possibility that a protein with esterase/lipa se activity plays a role in extracellular surfactant metabolism.