AMINE PRECURSOR UPTAKE AND DECARBOXYLATION - SIGNIFICANCE FOR PROCESSING OF THE RAT GASTRIN PRECURSOR

1. Conversion of prohormone precursors to smaller active products occu rs in secretory granules, which also has the capacity to concentrate b iogenic amines. We have examined how processing of the gastrin precurs or, progastrin, in rat antral mucosa is influenced by modulation of th e biogenic amine content of secretory grannies. 2. Newly synthesized p rogastrin-derived peptides in rat antral mucosa were labelled in vitro with (SO42-)-S-35 using a pulse-chase protocol and detected after imm unoprecipitation by HPLC with on-line liquid scintillation counting. S ecretory granule morphology was examined by electron microscopy. The e ffects of experimentally manipulating secretory granule pH and amine c ontent were examined. 3. The dopamine precursor L-beta-3,4-dihydroxyph enylalanine (L-DOPA) inhibited cleavage of S-35-labelled thirty-four a mino acid amidated gastrin, i.e. [S-35]G34, and of [S-35]G34 with COOH -terminal glycine, i.e. [S-35]G34-Gly, at a pair of lysine residues, b ut did not influence cleavage of progastrin at pairs of arginine resid ues. The effect of L-DOPA was reversed by reserpine, which inhibits th e amine-proton exchangers VMAT(1) and VMAT(2), and by carbidopa, which inhibits aromatic L-amino acid decarboxylase. 4. Treatments that rais e intragranular pH, e.g. the weak base chloroquine, the ionophore mone nsin and the vacuolar proton pump inhibitor bafilomycin A(1), had simi lar effects to L-DOPA. 5. Electron microscopical studies showed that t he electron-dense aggregates in gastrin cell secretory granules were l ost after inhibition of the vacuolar proton pump. Treatment with L-DOP A produced reserpine-sensitive dissipation of the electron-dense aggre gates, compatible with the idea that increased amine delivery raised i ntragranular pH. 6. The data suggest that the processes of amine precu rsor uptake, decarboxylation and sequestration in secretory granules a re associated with selective modulation of progastrin cleavage, possib ly by raising intragranular pH and thereby inhibiting pH-sensitive pro hormone convertases.