Affinity of phosphatidylcholine molecular species for the bovine phosphatidylcholine and phosphatidylinositol transfer proteins. Properties of the sn-1 and sn-2 acyl binding sites

Abstract

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphatidylcholine transfer protein from bovine liver contains highly unsaturated phosphatidylcholine species

“…In agreement with this study it was found that PC-TP prefers 1-palmitoyl-2-arachidonoyl-PC (C16:0/C20:4 PC) and 1-palmitoyl-2-docosahexanoyl-PC (C16:0/C22:6 PC) over any other naturally occurring PC species in vitro [10].…”

“…It is striking that the majority (57%) of the endogenous bound PC species contain fatty acyl chains (C20:4, C20:5 and C22:5) which are precursors in the prostaglandin/leukotriene biosynthesis [21]. This may indicate that PC-TP is involved in the delivery of these species to sites containing receptor-controlled phospholipases A2 [10]. According to this model PC-TP plays a role in signal transduction similar to what is observed for PI-TP [22].…”

“…So far, the preference of PC-TP for certain PC species has been investigated using in vitro transfer activity and binding assays [9,10]. In order to gain more insight into the physiological function of PC-TP, the molecular species composition of PC bound to PC-TP in vivo should be determined.…”

Clofibrate-Induced Relocation of Phosphatidylcholine Transfer Protein to Mitochondria in Endothelial Cells

“…In transferring PC between membranes, PC-TP prefers PC molecular species that have a palmitic acid on the sn-1 position and polyunsaturated fatty acid (PUFA) on the sn-2 position [42]. It is conceivable that relocation of PC-TP to mitochondria increases the amount of 16:0/PUFA-PC species in the mitochondrial membranes.…”

Determination of the Stability of the Noncovalent Phospholipid Transfer Protein−Lipid Complex by Electrospray Time-of-Flight Mass Spectrometry

“…Given that specific binding sites exist for the sn-1 and sn-2 acyl chains (29), it appears that the sn-1 saturated acyl chain dictates the stability of the complex more prominently than the sn-2 unsaturated acyl chain. Previously, by measuring the equilibration of PC between protein and membrane interface, PC-TP was found to more readily bind PC species carrying a C16:0 acyl chain than a C18:0 acyl chain (30). This difference in apparent affinity reflects the rates at which the PC monomer associates and dissociates from PC-TP and the membrane interface in the collision complex between PC-TP and the membrane.…”