Evidence that a mitogen-inducible prolactin-immunoreactive protein in rat spleen lymphocytes is aldolase A

This study characterizes several proteins in rat spleen lymphocyte lysates and conditioned medium that are recognized by antiserum to purified rat pituitary prolactin (PRL). One of these proteins, rat prolactin-immunoreactive protein (rPIP-43), has a relative molecular mass (Mr) of 43,000 and is strongly induced by mitogenic stimulation in spleen lymphocytes. A constitutively expressed protein of this size also was detected in the IM-9 human B lymphoblastoid cell line and the Nb2 rat T lymphoma cell line. The N-terminal amino acid sequence of rPIP-43 in spleen lymphocyte lysate was analysed and found to be identical with 25 residues at the N-terminus of the glycolytic enzyme aldolase A. In further experiments, the rPRL antiserum was evaluated for cross-reactivity with an aldolase A preparation and recognized a Mr 43,000 protein in rabbit muscle. Preabsorption of rPRL antiserum with rPRL was found to greatly decrease the intensity of staining of rPRL, aldolase A and rPIP-43. Preabsorption of antiserum with aldolase A had a similar, but less pronounced effect, with the aldolase A band and rPIP-43 being stained less intensely, while there was no effect on the intensity of staining of purified rPRL. Thus, data indicate that rPIP-43 is not a structural variant of PRL, but appears to be a different protein. These results have implications for the use of PRL antiserum to detect PRL in biological samples insofar as aldolase A is a ubiquitously expressed protein.