Purification and characterization of pheromaxein, the porcine steroid‐binding protein

Low molecular mass proteins are implicated in chemical communication throughout mammalian species, being involved in both perception and delivery of pheromonal compounds. In boars, pheromones are secreted in saliva to cause oestrous sows to take up the mating stance. These pheromones are the 16‐androstene steroids, 5α‐androsten‐3α‐ol and 5α‐androsten‐3‐one. The submaxillary glands of boars contain a low molecular mass protein, pheromaxein, which is capable of binding these 16‐androstene pheromones. Pheromaxein was purified, cloned and characterized. It was found to be a nonglycosylated heterodimeric protein, belonging to the secretoglobin superfamily and the major 16‐androstene‐binding protein present in submaxillary salivary glands of the boar. One subunit, pheromaxein A, was found to be homologous to prostatein peptides, C1 and C2 and lipophilin A and B, whereas the other subunit, pheromaxein C, was homologous to prostatein peptide C3 and lipophilin C. Transcription of pheromaxein A was limited to the prostate and submaxillary salivary glands from both the boar and sow, whereas transcription of the other subunit, pheromaxein C, was more widespread. This is similar to the transcription distribution of lipophilin in humans. Many isoforms of pheromaxein were found to exist, with a molecular mass range of 17 415–18 159 Da; these are probably products of a multigene family. Post‐translational modifications, to generate mature pheromaxein isoforms, probably include C‐terminal cleavage of pheromaxein A, followed by additional modifications.