Pulmonary surfactant-associated protein A enhances the surface activity of lipid extract surfactant and reverses inhibition by blood proteins in vitro

Abstract

Although a monolayer of dipalmitoylphosphatidylcholine, the major component of pulmonary surfactant, is thought to be responsible for the reduction of the surface tension at the air-liquid interface of the alveolus, the participation of unsaturated and anionic phospholipids and the three surfactant-associated proteins is suggested in the generation and maintenance of this surface-active monolayer. We have examined the effects of surfactant-associated protein A (SP-A) purified from bovine lavage material on the surface activity of lipid extract surfactant (LES), an organic extract of pulmonary surfactant containing all of the phospholipids and SP-B and SP-C, but lacking SP-A. Measurements of the surface tension during dynamic compression were made on a pulsating bubble surfactometer. Addition of SP-A to LES reduces the number of pulsations required to attain surface tensions near zero at minimum bubble radius. This increase in surface activity is dependent upon the presence of Ca2+ in the assay mixture. Maximal enhancement is observed at or below 1% of the lipid concentration (w/w). The addition of two blood proteins, fibrinogen and albumin, at physiological concentrations to LES causes severe inhibition of surface activity. Addition of SP-A in the presence of Ca2+ completely counteracts the inhibition by fibrinogen. The amount of SP-A required for full reversal of this inhibition was less than 0.5% of the lipid concentration. Complete reversal of inhibition by albumin was also observed, even though there was a approximately 5000-fold molar excess of inhibitor. Addition of lysophosphatidylcholine also inhibits LES; however, SP-A has no effect on this inhibition.

Protective effects of elevated endogenous surfactant pools to injurious mechanical ventilation

“…For example, endogenous pools contain all the components of surfactant, whereas the currently available exogenous surfactant preparations lack several components, most notably surfactant protein A (SP-A) (18). This protein has important host defense functions and protects surfactant against inhibition by serum proteins (5,20); SP-A may therefore play an important role in the prevention of VILI and other lung injuries. Increasing endogenous pools may also provide a more uniform distribution of newly secreted surfactant, something that is difficult to accomplish via instillation of exogenous surfactant preparations given the current methods of administration (12,13).…”

Pulmonary Surfactant Protein-A (SP-A) Restores the Surface Properties of Surfactant after Oxidation by a Mechanism That Requires the Cys6 Interchain Disulfide Bond and the Phospholipid Binding Domain

“…Whether SP-A reversal of the effects of surfactant oxidation requires SP-B or SP-C must be determined through reconstitution studies. In addition to counteracting effects of surfactant PL oxidation, SP-A can also counteract inhibitory effects of non-surfactant proteins (17,18). Studies comparing SP-A domain and structural requirements for these two prosurfactant activities are currently in progress.…”

“…In vitro studies indicate that the most abundant surfactantassociated protein, SP-A, facilitates the surface tension-lowering properties of surfactant PLs (16 -18) and counteracts protein inactivation of surfactant in vitro (17,19).…”

Host defence capacities of pulmonary surfactant: evidence for ‘non‐surfactant’ functions of the surfactant system

“…SP-A regulates the amount of surfactant in alveoli through a receptor-mediated process on type I1 pneumocytes [51-531, accelerates the adsorption of surfactant to the interface [54,55] and augments macrophage-dependent defence reactions [56]. It might also prevent surfactant inhibition through non-surfactant proteins [57].…”