Liver carboxylesterase cleaves surfactant protein (SP-) B and promotes surfactant subtype conversion

Conversion of the biophysically active large surfactant aggregate subtype of alveolar surfactant into the less surface active small surfactant aggregates occurs in vitro and in vivo, possibly in dependency of a carboxylesterase, entitled surfactant convertase. The substrate has yet not been safely identified. Utilizing the in vitro cycling assay we investigated conversion of an organic rabbit lavage extract reconstituted with SP-A. Porcine liver carboxylesterase, which is closely related to surfactant convertase, induced subtype conversion to a similar degree as compared with native lavage fluid containing endogenous convertase. In addition, we asked for cleavage products of SP-B and identified a ∼12 kDa band upon cycling with liver carboxylesterase, having the same N-terminus as mature SP-B. A band of same molecular weight was found in native lavage fluid after in vitro conversion mediated by the endogenous convertase. We conclude that SP-B plays a pivotal role during subtype conversion and represents the substrate for surfactant convertase.