Partial purification and functional properties of an endoprotease from bovine neurosecretory granules cleaving prooxytocin/neurophysin peptides at the basic amino acid doublet

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Abstract
An enriched preparation of neurosecretory granules from bovine pituitary neural lobes was used as a source of processing enzymes possibly involved in the cleavage of the proocytocin/neurophysin precursor. A synthetic eicosapeptide reproducing the entire (1-20) sequence of the NH2-terminal domain of the bovine ocytocin/neurophysin precursor was used as a substrate to monitor an endoprotease activity cleaving at the Lys11.sbd.Arg12 doublet. The 58-kDa endoprotease detected in the lysate of neurohypophyseal granules produced a single cleavage, after the doublet, at the Arg12.sbd.Ala134 peptide bond. This endoprotease with pHi 6.9 and 7.2 exhibits maximal activity at pH around neutrality (7.0) and was strongly inhibited by divalent cation chelating agents [ethylenediaminetetraacetic acid and ethylene glycol bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid]and to some extent by p-(chloromercuri)benzoate and p-(chloromercuri)benzenesulfonic acid, while phenylmethanesulfonyl fluoride and pepstatin were not active. This endoprotease action was sensitive to any modification of the substrate at either basic amino acid of the doublet since replacement of either L-Lys11 or L-Arg12 by D-Lys or D-Arg and by L-Nle abolished the cleavage reaction. In contrast, reversal of the polarity of the doublet in [Arg11,Lys12]proocytocin/neurophysin(1-20) had no effect on the mode of endoproteolytic cleavage as well as modifications of Gly10 (replaced by Ala10). It is concluded that the selectivity of this endoprotease, which may be involved in the primary event occurring in prooocytocin/neurophysin processing, is strictly dependent upon the integrity of the basic doublet but that other parameters determined by the amino acid sequence around this doublet may play an important role.

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