An albumin-associated PLA2-like activity inactivates surfactant phosphatidylcholine secreted from fetal type II pneumocytes

Abstract

Type II pneumocytes are responsible for the synthesis and secretion of pulmonary surfactant, which reduces surface tension in lung alveoli, thus decreasing their tendency to collapse during expiration. For this effect to be sustained, the integrity of the surface-active components of surfactant must be maintained. This study has shown that, when cultured type II pneumocytes are exposed to lipoprotein-free serum (LFS), the level of lyso-phosphatidylcholine (lyso-PC) in the secreted surfactant phospholipids is markedly elevated with a concomitant decline in the level of phosphatidylcholine (PC). This effect is the result of hydrolysis of surfactant PC by a phospholipase A2(PLA2)-like activity present within serum. Anion-exchange chromatography, gel filtration chromatography and preparative electrophoresis of human LFS have shown that this PLA2-like activity coelutes with albumin and is biochemically distinct from the secretory form of PLA2. Furthermore, specific inhibitors of PLA2such as p-bromophenacyl bromide, aristolochic acid, and palmitoyl trifluoromethyl ketone do not inhibit this activity of serum. Commercially purified human serum albumin fraction V and recombinant human serum albumin (rHSA) are almost as effective as LFS in enhancing the level of lyso-PC in the media. The latter finding implies that rHSA directly generates lyso-PC from secreted PC and suggests that this PLA2-like activity may be an intrinsic attribute of albumin.