Isolation and identification of a mouse brain protein recognized by antisera to heart fatty acid-binding protein

Although a novel brain-specific fatty acid-binding protein (B-FABP) was rec ently cloned, the identity of a second fatty acid-binding protein detected with antibodies to the heart (H-FABP) has not been clearly resolved. The pr esent investigation, using matrix-assisted laser desorption mass spectromet ry, showed that this protein was a form of H-FABP whose N-terminal amino ac id was neither methionine nor was it acetylated. Furthermore, isoelectric f ocusing revealed two major isoforms, a major band pI 7.4 and a minor band p I 6.4, in a distribution pattern opposite to that observed for H-FABP in th e heart. Tryptic peptide mass maps of the in-gel digested SDS polyacrylamid e gel electrophoresis protein bands showed that the two isoforms differed o nly in a single peptide corresponding to residues 97-106 of the heart H-FAB P sequence. This peptide had an [M + H](+) ion of either 1205.62 (pI 7.4) o r 1206.53 (pI 6.4), consistent with a single amino acid substitution, Asp98 or Asn98. Whereas it is well established that both H-FABP and B-FABP inter act with polyunsaturated fatty acids, we showed that they also significantl y alter plasma membrane cholesterol dynamics in a manner opposite to that o f another brain lipid-binding protein, sterol carrier protein-2. In summary , the data demonstrated for the first time that the H-FABP from brain, whil e nearly identical to H-FABP from heart, differed significantly in isoform distribution and in amino terminal structure from heart H-FABP. This sugges ts that the brain and heart H-FABP may not necessarily function identically in these tissues.