An in vitro reversible interconversion of rat liver fatty acid binding protein having different isoelectric points by virtue of the fatty acid content.

Rat liver fatty acid binding protein (FABP) was purified to homogeneity by procedures including Sephadex G-100 and DEAE-cellulose column chromatographies. FABP was resolved into two major peaks, A and B, by the first DEAE-cellulose column chromatography. Each of these two fractions exhibited apparent homogeneity upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate with a molecular weight of 14,000 Da and amino acid analysis of these fractions has revealed that they are virtually identical or closely resemble each other. However, their fatty acid content was significantly different and heterogeneity was clearly demonstrated in the patterns of isoelectric focusing. In this communication, a single isoform (pI 5.0) from peak B FABP was further purified by successive DEAE-cellulose column chromatography and used as the final preparation. When the final FABP was partly freed of fatty acids by a mild delipidation technique using Lipidex 1,000, the pI shifted upward from 5.0 to 7.0. However, the pI of the delipidated FABP returned to its original pI of 5.0 after recombining fatty acids. These in vitro manipulations of bound fatty acid content made clear its possible cause of the microheterogeneity of FABP.