GAP-releasing enzyme is a member of the pro-hormone convertase family of precursor protein processing enzymes.

The recent discovery of mammalian endoproteinases which show extensive sequence homology with the yeast Kex 2 gene product (kexin) has lead to the hypothesis that processing enzymes of pro-hormone precursor proteins belong to a family of calcium dependent, subtilisin-like serine proteinases. We previously showed that hypothalamic GAP-releasing enzyme shares these characteristics and possesses the requisite specificity to be considered as a processing enzyme of progonadotropin releasing hormone (pro-GnRH) precursor protein. Thus, GAP-releasing enzyme (and other non-related proteins) were tested for their immunological reactivity with antisera raised against pituitary pro-hormone convertase 1/3 (PC1/3) and insulinoma PC2. On the basis of indirect enzyme-linked immunosorbent (ELISA) and Western blot assays, GAP-releasing enzyme is now shown to be immunologically related to PC1/3. We can conclude that GAP-releasing enzyme is also likely to be a member of the pro-hormone convertase family and should be considered the physiologically relevant processing enzyme of pro-GnRH. It is possible that GAP-releasing enzyme represents bovine hypothalamic PC1/3.