Biophysical characterization and modeling of lung surfactant components

The present study characterizes the dynamic interfacial properties of calf lung surfactant (CLS) and samples reconstituted in a stepwise fashion from phospholipid (PL), hydrophobic apoprotein (HA), surfactant apoprotein A (SP -A), and neutral lipid fractions. Dipalmitoylphosphatidylcholine (DPPC), th e major PL component of surfactant, was examined for comparison. Surface te nsion was measured over a range of oscillation frequencies (1-100 cycles/mi n) and bulk phase concentrations (0.01-1 mg/ml) by using a pulsating bubble surfactometer. Distinct differences in behavior were seen between samples. These differences were interpreted by using a previously validated model o f surfactant adsorption kinetics that describes function in terms of 1) ads orption rate coefficient (k(1)), 2) desorption rate coefficient (k(2)), 3) minimum equilibrium surface tension (gamma*), 4) minimum surface tension at film collapse (gamma(min)), and 5) change in surface tension with interfac ial area for gamma < gamma* (m(2)). Results show that DPPC and PL have k(1) and k(2) values several orders of magnitude lower than CLS. PL had a gamma (min), of 19-20 dyn/cm, significantly greater than CLS (nearly zero). Addit ion of the HA to PL restored dynamic interfacial behavior to nearly that of CLS. However, m(2) remained at a reduced level. Addition of the SP-A to PL + HA restored m(2) to a level similar to that of CLS. No further improveme nt in function occurred with the addition of the neutral lipid. These resul ts support prior studies that show addition of HA to the PL markedly increa ses adsorption and film stability. However, SP-A is required to completely normalize dynamic behavior.