Isolation and characterization of fatty acid binding proteins from mammary tissue of lactating rats

A protein fraction with fatty acid binding activity has been isolated from mammary tissue from lactating rats by a process involving DEAE-cellulose ion-exchange chromatography, heat treatment, CM-cellulose ion-exchange chromatography and finally ammonium sulphate precipitation. The purified fraction migrated as a single band on SDS/polyacrylamide-gel electrophoresis with an apparent molecular mass of 14400. However, when this protein fraction was electrophoresed under non-dissociating conditions, two species were observed in a 4:1 ratio. The two components were separated using h.p.l.c. Both bind fatty acids and appear to have similar amino acid compositions although exhibiting different pI values of 4.8 and 4.9. The mammary fatty acid binding proteins appear to be very similar to the fatty acid binding protein isolated from rat heart based on the electrophoretic mobilities and amino acid composition. The major mammary form (pI 4.9) has been partially sequenced and the amino acid sequences obtained can be aligned with 67 residues of the revised rat heart amino acid sequence [Heuckeroth, Birkenmeier, Levin & Gordon (1987) J. Biol. Chem. 262, 9709-9717]. Both mammary species also showed immunochemical identity to rat heart fatty acid binding protein when tested with an anti-serum raised against the heart protein. Anti-sera raised against the minor mammary form (pI 4.8) specifically precipitated this form under non-denaturing conditions but both forms after they had been denatured. Quantitative immunoassays using the anti-(heart fatty acid binding protein) serum showed that concentrations of the fatty acid binding proteins present in mammary cytosols increase during lactation and increase further after feeding a high-fat diet.