Abstract
The binding of lysophospholipids to rat liver fatty acid-binding protein (FABP) and to BSA and human serum albumin was investigated by using competitive displacement fluorescence assays by monitoring the displacement of the fluorescent fatty acid probe 11-(dansylamino)undecanoic acid (DAUDA). In addition, direct binding assays using changes in tryptophan fluorescence were possible with albumin. Liver FABP was able to bind a range of lysophospholipids, oleoyl-lysophosphatidic acid (lysoPA), oleoyl-lysophosphatidylcholine (lysoPC), oleoyl-lysophosphatidylethanolamine (lysoPE) and oleoyl-lysophosphatidylglycerol, with similar affinity and a Kd of about 1 microM. Liver FABP was also able to bind lysophospholipids generated by the action of phospholipase A2 or phospholipase A1 (triacylglycerol lipase) on phospholipid vesicles. A possible physiological role for liver FABP in lysophospholipid metabolism within the cell is discussed. Albumin was shown to bind lysoPA with higher affinity than either lysoPC or lysoPE, and the initial minimal DAUDA displacement by lysoPA indicated that lysoPA was binding to the primary high-affinity fatty acid-binding sites on albumin and that, like oleic acid, about 3 mol of ligand/mol was bound to these sites. Kd values in the microM range were indicated for lysoPC and lysoPE, whereas, by comparison with oleic acid, the Kd for lysoPA was significantly lower and high-affinity binding in the nM range was indicated. Overall, the data suggest that, because of structural similarity, lysoPA binds to albumin in a similar manner to long-chain fatty acids.