ProSAAS processing in mouse brain and pituitary

ProSAAS is a newly discovered protein with a neuroendocrine distribution ge nerally similar to that of prohormone convertase 1 (PC1), a peptide-process ing endopeptidase. Several proSAAS-derived peptides were previously identif ied in the brain and pituitary of the Cpe(fa)t/Cpe(fat) mouse based on the accumulation of C-terminally extended peptides due to the absence of enzyma tically active carboxypeptidase E, a peptide-processing exopeptidase. In th e present study, antisera against different regions of proSAAS were used to develop radioimmunoassays and examine the processing profile of proSAAS in wild type and Cpe(fat)/Cpe(fat) mouse tissues following gel filtration and reverse phase high performance liquid chromatography. In wild type mouse b rain and pituitary, the majority of proSAAS is processed into smaller pepti des. These proSAAS derived peptides elute from the reverse-phase column in the same positions as synthetic peptides that correspond to little SAAS, PE N, and big LEN. Mass spectrometry revealed the presence of peptides with th e expected molecular masses of little SAAS and big LEN in the fractions con taining immunoreactive peptides. The processing of proSAAS is slightly impa ired in Cpe(fat)/Cpe(fat) mice, relative to wild-type mice, leading to the accumulation of partially processed peptides. One of these peptides, the C- terminally extended farm of PEN, is known to inhibit PC1 activity and this could account for the reduction in enzymatically active PC1 seen in Cpe(fat )/Cpe(fat) mice. The observation that little SAAS and big LEN are the major forms of these peptides produced in mouse brain and pituitary raises the p ossibility that these peptides function as neurotransmitters or hormones.