Fig. 5.

Concentration-dependent binding of SP-A to type II cells. Increasing concentrations of ¹²⁵I-SP-A were incubated with pneumocyte cells plated on 12-well plates at 4°C for 1 h followed by harvest with 2 N NaOH. A: SP-A binding to type II cells. Total binding (closed circles) was measured in the presence of calcium. Specific binding (open circles) was determined by the “slope-peeling” method (16) where the nonspecific portion of the binding curve is subtracted from the total binding curve. Data are means ± SE of 3–8 experiments performed in duplicate or triplicate expressed as a percentage of the nanograms of SP-A per milligram of cell protein bound at 1 μg of ¹²⁵I-SP-A/ml. Total binding at 1 μg SP-A/ml (100% value) was 104.5 ± 30.4 ng SP-A/mg cell protein (mean ± SE, n = 8) with a range of 40–279 ng SP-A/mg cell protein. B: Klotz plot of the specific binding of SP-A. The log of the concentration of free SP-A (μg/ml) is plotted vs. the amount of SP-A specifically bound (ng SP-A/mg cell protein) to the cells normalized to the mean binding value at 1 μg SP-A/ml for the 8 experiments (104.5 ng SP-A/mg cell protein).