SURFACTANT CONVERTASE ACTION IS NOT ESSENTIAL FOR SURFACTANT FILM FORMATION

A serine-active enzyme, ''surfactant convertase,'' is required for the conversion of surfactant from the tubular myelin (TM) form to the sma ll vesicular (SV) form. This transformation involves at least two step s, the conversion of TM to a surface-active film at the air-fluid inte rface and the reorientation of the film into the surface-inactive SV f orm; we asked if convertase was required for the first of these steps. Rat and mouse TMs were pretreated with diisopropyl fluorophosphate (D FP) to inactivate endogenous convertase activity or with vehicle and t hen were analyzed for their ability to lower surface tension in vitro as an index of the conversion of TM to a surface film. DFP pretreatmen t did not alter the ability of TM preparations to lower surface tensio n, as assessed by pulsating bubble, and it did not affect the behavior of TM in a surface balance. In an experiment designed to test the abi lity of TM to feed a surface film to exhaustion, TMs that had been pre treated with DFP or vehicle performed similarly. These experiments sho w that convertase activity is not required for the conversion of TM to a monolayer and suggest, instead, that convertase acts at a post surf ace film stage.