Pro-xenopsin(s) in vesicles of mammalian brain, liver, stomach and intestine is apparently released into blood and cerebral spinal fluid

Abstract

Mammalian pro-xenopsins (proXP), proteins (such as α-coatomer) that yield XP-related peptides when digested by pepsin-related proteases, are ubiquitously distributed in rats, with highest concentrations in liver and gastrointestinal tissues. Here, the cellular and subcellular distributions of canine and rat proXP were determined in brain, liver, stomach and intestine. Elutriation and percoll density centrifugation of collagenase-dispersed cells demonstrated that proXP was primarily associated with hepatocytes in liver, chief and parietal cells in stomach and endocrine/exocrine cells in intestine. When fragmented cells were subjected to differential centrifugation, ≅85% of proXP was associated with particulate fractions and only ≅15% was cytosolic. Sucrose-gradient centrifugation of crude mitochondrial preparations (P2 pellets) for liver, stomach and intestine demonstrated that proXP was localized to vesicles (density, ≅1.19; size, 80–400 μm), which contained material of variable electron density. In isotonic homogenates of brain, proXP migrated primarily with synaptosomes (density, ≅1.15) which contained vesicles (size, 50–100 μm). During HPLC-sizing and ion exchange chromatography, proXP gave at least three components, the major one being an anionic 140-kDa protein. ProXP-like activity was found in human and rat blood, human cerebral spinal fluid and in contents of the gastrointestinal lumen. These results are consistent with the idea that these vesicle-associated protein(s) could be released during endocrine and/or exocrine secretion and serve as precursors to XP-related peptides.