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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Pituitary enzyme conversion of putative synthetic oxytocin precursor intermediates.

Neurosecretory granule lysate from bovine posterior pituitary was shown to contain both carboxypeptidase B and amidating activities. The former sequentially releases COOH-terminal basic residues from the oxytocin biosynthetic precursor fragment oxytocinyl-GKR (CYIQNCPLGKR) to form oxytocinyl-GK and then oxytocinyl-G. The amidating enzyme converts the resulting oxytocinyl-G into oxytocin (CYIQNCPLG-NH2). The carboxypeptidase B was separated from a less specific carboxypeptidase present in granule lysate by gel filtration on Sephacryl S-300. Percoll density gradient centrifugation (after preliminary differential centrifugation) also yielded granule fractions enriched in the specific carboxypeptidase B activity. The carboxypeptidase B which converts the oxytocinyl peptides showed a fairly sharp pH dependence with an optimum of 5.5-6, was activated by cobalt ion, and was inhibited by cupric ion, EDTA, and a thiol protease inhibitor, p-chloromercuribenzoate. The amidating activity which converts oxytocinyl-G to oxytocin was competed by degradation due to proteases and/or peptidases present in lysate of Percoll gradient-derived granules. Oxytocinyl-GKR was shown by analytical affinity chromatography to bind noncovalently to neurophysin with an affinity close to that of mature oxytocin. This binding activity and the observation of carboxypeptidase B activity in the presence of large concentrations of neurophysin are consistent with the view that the exoproteolytic processing and amidation steps which occur after initial endoproteolysis of pro-oxytocin/neurophysin likely take place on oxytocin intermediate peptides which are bound in noncovalent complexes with the neurophysin domain from the precursor.[1]


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