Calcium and dithiothreitol dependent conformational changes in beta-sheet structure of collagenase resistant fragment of human surfactant protein A.

Ca2+ dependent conformational change of collagenase resistant fragment (CRF) of human surfactant protein A (SP-A) was studied by measurements of the far UV circular dichroism spectrum. The spectrum was altered by Ca2+ and DTT. The beta-sheet content was decreased by the addition of Ca2+ from 28.1 to 26.6%. On the other hand, the beta-sheet content was increased in the presence of dithiothreitol from 28.1 to 36.0%, and decreased by the addition of Ca2+ from 36.0 to 30.5%. The total Ca2+ concentration required for half maximal change of the ellipticity at 220 nm was estimated to be 30 microM both in the presence and absence of dithiothreitol. One of the functions of SP-A, enhancement of phospholipid uptake by alveolar type II cells, was abolished by the addition of 2-mercaptoethanol. These results strongly indicate a relationship between the conformation of CRF and SP-A functions.