Conformation change of the intestinal calcium-binding protein induced by phospholipids in the presence and absence of Ca2+

Effects of phospholipids (PL's) and lyso-PL's on the conformation of the porcine intestinal calcium-binding protein (CaBP) were studied fluorometrically with 1-(dimethylamino)naphthalene-5-sulfonyl-(DNS-) labeled CaBP. The fluorescence intensity of DNS-labeled CaBP was much higher in the presence of excess EGTA than in its Ca2+-bound state. In the absence of free Ca2+ (with 1 mM EGTA) the fluorescence of the labeled CaBP was greatly enhanced by addition of lysophosphatidylcholine (lyso-PC), lysophosphatidylserine (lyso-PS), or lysophosphatidylinositol (lyso-PI). With addition of 25 microM Ca2+ the enhancement of the fluorescence by these lyso-PL's was depressed; especially that due to lyso-PC became small. Lysophosphatidylethanolamine (lyso-PE), phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylethanolamine (PE), and mono- and dipalmitoylglycerols had no or much less effect on the fluorescence in the presence and absence of Ca2+. Lyso-PC attenuated in a concentration-dependent manner the quenching of the fluorescence of the DNS-CaBP by high temperatures and increase of ionic strength in the presence of EGTA. Lyso-PL's generally protected the CaBP from digestion with proteases in the presence of EGTA. These experimental results suggest that particular lyso-PL's have Ca2+-sensitive interaction with the porcine CaBP and induce conformation change of the CaBP molecules.