Permeability of zymogen granule membrane to protein

Conservation of digestive enzymes

The traditional understanding is that an entirely new complement of digestive enzymes is secreted by the pancreas into the small intestines with each meal. This is thought to be necessary because, like food itself, these enzymes are degraded during digestion. In this review we discuss experiments that bring this point of view into question. They suggest that digestive enzymes can be absorbed into blood, reaccumulated by the pancreas, and reutilized, instead of being reduced to their constituent amino acids in the intestines. This is called an enteropancreatic circulation of digestive enzymes.

Chemiosmotic control of renin release from isolated renin granules of rat kidneys

1. Renin-containing granules were isolated, characterized, and used to gain insight into a possible chemiosmotic mechanism of renin secretion. 2. Renin granules were obtained by a modification of the sucrose gradient method, which yielded a 67-fold purification of renin granules as assessed by marker enzymes, or a modification of the Percoll gradient, which yielded a 230-fold enrichment of renin granules. 3. Granular renin content was increased by chronic sodium deprivation and hypophysectomy. 4. Renin release from granules was inversely related to osmotic strength (150-900 mosmol l-1). pH had a biphasic effect on renin release, with greater stimulation at both acidic (pH 5) and alkaline (pH 8 and 9) pH. The pH effect was dependent on Cl-; raising Cl- stimulated release. This effect was abolished by-oligomycin and N,N'-dicyclohexylcarbodiimide (DCCD) at pH 5, but not at pH 8; the effect was enhanced by NH4+. 5. Either valinomycin or carbonyl cyanide m-chlorophenylhydrazone (CCCP) alone was without effect; but in combination they caused a potent stimulation at all pHs. Nigericin stimulated renin release at all pHs, but its effect required K+. 6. Raising K+ stimulated renin release from granules, whereas raising Na+ was without effect. Lowering Ca2+ below 10(-6) M significantly stimulated renin release. 7. Taken together, the evidence is consistent with the chemiosmotic hypothesis for the control of renin release from granules and may have some implications for the regulation of renin secretion from juxtaglomerular cells.

Effects of a cytosolic protein on the interaction of rat pancreatic zymogen granules in vitro

Photon correlation spectroscopy has been used to study the kinetics of aggregation of isolated rat pancreatic zymogen granules in vitro by monitoring time-dependent changes in mean particle size derived from the photon count autocorrelation function, g2(tau). Isolated granules were stable in isotonic sucrose (pH 5.4-7.0). At pH 6.0 they maintained a mean diameter of 1225 +/- 18 nm with a polydispersity index of 0.199 +/- 0.007. The mean granule diameter showed a limited decrease (approx. 20%) with increasing pH within the range 5.4-7.0, but the polydispersity index was unaltered. At pH greater than 7.0 granule instability was indicated by a rapid reduction in total photon counts. In solutions of monovalent cations ([M+] greater than 10 mM) and divalent cations ([M2+] greater than 0.5 mM) zymogen granules aggregated at a rate dependent upon both ion and granule concentration. These effects were consistent with the bimolecular nature of the interaction mechanism and were clearly distinguishable from the limited size changes associated with osmolarity. At concentrations of Na+ or K+ salts greater than 50 mM granule aggregation was accompanied by anion-dependent solubilisation. A soluble protein fraction separated from the pancreatic acinar cell cytosol by gel filtration reduced the mean diameter and polydispersity index of zymogen granules suspended in isotonic sucrose, inhibited cation-induced aggregation and stabilised granules to solubilisation induced by raising pH greater than 7.0 or exposure to high ionic strength media. The inhibitory effects of this protein were apparent at concentrations less than or equal to 10 micrograms X ml-1 (i.e. at inhibitor: granule protein ratios less than 1:20) and could not be mimicked by bovine serum albumin, the Ca2+-binding proteins calmodulin and troponin C (less than or equal to 100 micrograms X ml-1), nor the highly negatively charged polymer polyglutamate (less than or equal to 10 micrograms X ml-1). Inhibitory activity was also absent from fractions of rat liver cytosol prepared identically to pancreatic acinar cytosol. These observations are consistent with the presence in pancreatic acinar cells of a specific cytosolic granule stabilisation factor (or factors) that normally restricts zymogen granule interaction and may therefore play an important role in the regulation of granule mobility and exocytosis.