We are characterizing the alternatively spliced human peptidylglycine alpha-amidating monooxygenase (hPAM)-encoding mRNA transcripts expressed by human cells. Reverse transcription coupled to the polymerase chain reaction (RT-PCR) has been used to identify four alternatively spliced variants that differ in the region joining the two catalytic domains. Two of the transcripts represent previously reported splice variants differentiated by the presence (hPAM-A) or absence (hPAM-B) of a 321-nucleotide (nt) linker (optional exon A) which in the rat produce functionally distinct enzymes. Different mRNAs represent two splice variants, hPAM-C and hPAM-D, that show the presence of an exon unreported for PAM in any other species. This new exon, designated exon C, is 54 nt in length, encodes an 18-amino-acid (aa) peptide containing a conserved dibasic aa endoproteolytic processing motif, and is located 3' of exon A in human genomic DNA. We propose that cell-specific regulation of mRNA splicing would provide a mechanism for control of prohormone activation by these variants of the PAM enzyme.