Determination of the acyl chain specificity of the bovine liver phosphatidylcholine transfer protein. Application of pyrene-labeled phosphatidylcholine species

Abstract

The phosphatidylcholine transfer protein from bovine liver has specific binding sites for the sn-1 and sn-2 acyl chains of the phosphatidylcholine molecule [Berkhout, T. A., Visser, A. J. W. G., and Wirtz, K. W. A. (1984) Biochemistry 23, 1505-1513]. In the present study, we have investigated the properties of these binding sites by determining both binding and transfer of several sets of pyrenlyphosphatidylcholine species. These sets consisted of positional isomers in which the length of the pyrene-labeled acyl chain (i.e., 5-13 methylene units) or of the unlabeled saturated acyl chain (i.e., 9-19 methylene units) was varied in either the sn-1 or the sn-2 position. Binding studies showed that there was a considerable discriminantion between positional isomers with the higher affinity observed for those lipids that carry the pyrenyl chain in the sn-2 position. In addition, the affinity is markedly dependent on the length of the acyl chains; pyrenyl acyl chains of 9 and 11 methylene units and the palmitoyl chain provided the most efficient binding. The affinity of the transfer protein for the strongest bound pyrene lipid was approximately 2.5 times higher than for an average egg phosphatidylcholine molecule. In general the transfer studies were in agreement with the binding data. However, with some short-chain derivatives, transfer rates were faster than expected on the basis of the binding data. This emphasizes the importance of kinetic factors (i.e., activation energy) in the transfer process. The rates of spontaneous transfer decreased montonically with increasing chain length and were very similar for all positional isomer pairs studied. This strongly suggests that the discrimination between isomers in the protein-mediated transfer reflects the structure of the lipid binding site rather than differences in the physical properties of the isomers. In conclusion, the present data strongly support the presence of separate binding sites in the transfer protein for sn-1 and sn-2 chains of phosphatidylcholine and indicate that these binding sites have considerably different acyl chain specificity.