AFFINITY OF PHOSPHATIDYLCHOLINE MOLECULAR-SPECIES FOR THE BOVINE PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLINOSITOL TRANSFER PROTEINS - PROPERTIES OF THE SN-1 AND SN-2 ACYL BINDING-SITES

Both the phosphatidylcholine transfer protein (PC-TP) and the phosphatidylinositol transfer protein (PI-TP) act as carriers of phosphatidylcholine (PC) molecules between membranes. To study the structure of the acyl binding sites of these proteins, the affinity of 32 distinct natural and related PC molecular species was determined by using a previously developed fluorometric competition assay. Marked differences in affinity between species were observed with both proteins. Affinity vs lipid hydrophobicity (determined by reverse-phase HPLC) plots displayed a well-defined maximum indicating that the acyl chain hydrophobicity is an important determinant of binding of a phospholipid molecule by these transfer proteins. However, besides the overall lipid hydrophobicity, steric properties of the individual acyl chains contribute considerably to the affinity, and PC-TP and PI-TP respond differently to modifications of the acyl chain structure. The affinity of PC-TP increased steadily with increasing unsaturation of the sn-2 acyl moiety, resulting in high affinity for species containing four and six double bonds in the sn-2 chain, whereas the affinity of PI-TP first increased up to two to three double bonds and then declined. These data, as well as the distinct effects of sn-2 chain double bond position and bromination, indicate that the sn-2 acyl chain binding sites of the two proteins are structurally quite different. The sn-1 acyl binding sites are dissimilar as well, since variation of the length of saturated sn-1 chain affected the affinity differently. The data are discussed in terms of the structural organization of the sn-1 and sn-2 acyl binding sites of PC-TP and PI-TP. The major physiological implication of the present study is that in vivo all the major mammalian PC species should be transferred by PC-TP and PI-TP albeit at greatly variable rates. This conclusion may be extended also to the molecular species of PI, since the acyl chains of PC and PI probably share common binding sites in PI-TP [van Paridon, P. A., Gadella, T. W. J., Somerharju, P. J., & Wirtz, K. W. A. (1988) Biochemistry 27, 6208-6214].© 1990, American Chemical Society. All rights reserved.