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In the course of our studies on structure/function relationships of fatty‐acid‐binding proteins, we reported earlier that the two isoforms of the 15‐kDa cardiac fatty‐acid‐binding protein (cFABP) from bovine heart only differ in one position; Asn98 in pl 5.1‐cFABP; Asp98 in pI 4.9‐cFABP [Unterberg, C., Börchers, T., Højrup, P., Knudsen, J. and Spener, F. (1990) J. Biol. Chem. 265, 16 255–16 261]. In the present study, we elucidate the origin for this heterogeneity. Isoelectric focusing analysis of immunoprecipitated in vitro translation products from total mRNA and positivehybrid‐selected cFABP/mRNA revealed two l‐[35S]methionine‐labeled proteins corresponding to pI 5.1‐cFABP and pI 4.9‐cFABP. In a control experiment, recombinant mRNA derived from cDNA encoding pI 5.1‐cFABP was translated and produced only pI 5.1‐cFABP as shown by isoelectric focusing of the translation products. We could observe co‐translational acetylation but not posttranslational deamidation of the cFABP isoforms. Taken together, our results demonstrate that the isoforms of cardiac fatty‐acid‐binding protein found in bovine heart are coded by distinct mRNA species.

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