The mechanism of enzyme secretion by the cell I. Storage of amylase in the zymogen granules of the rat-parotis gland

Ca(2+)-independent fusion of secretory granules with phospholipase A2-treated plasma membranes in vitro

The fusion of secretory granules with plasma membranes prepared from rat parotid gland was studied in vitro to clarify the mechanism of exocytosis. Fusion of the granules with plasma membranes was measured by a fluorescence-dequenching assay with octadecyl rhodamine B, and release of amylase was also measured to confirm the fusion as a final step of the secretory process. Plasma membranes that had been pretreated with porcine phospholipase A2 (PLA2) in the presence of 20 microM Ca2+ fused with the granules within 30 s, and induced amylase release by reacting with the membranes of granules, whereas without this pretreatment they had no significant effect. The fusion process accompanied by amylase release was induced in the presence of 10 mM EGTA, and therefore was apparently Ca(2+)-independent. On the other hand, the presence of EGTA or 100 microM quinacrine, an inhibitor of PLA2, during treatment of plasma membranes with PLA2 inhibited their fusogenic activity, suggesting the importance of activation of PLA2. Arachidonic acid and linoleic acid were released from the plasma membranes during the PLA2 treatment. The presence of albumin, an adsorbent of fatty acids, during the treatment also inhibited the activity. Pretreatment of the membranes with arachidonic acid or linoleic acid did not have any effect, but the presence of exogenously added arachidonic acid during PLA2 treatment enhanced the membrane-fusion-inducing effect of PLA2. Pretreatment of the membranes with lysophosphatidylcholine induced fusogenic activity. These findings suggest that the conformational change in the plasma-membrane phospholipids induced by PLA2 and the presence of arachidonic acid or linoleic acid produced by PLA2 are important in the process of fusion of secretory granules with the plasma membranes of rat parotid acinar cells and that the fusion process itself is independent of Ca2+.

Diurnal variation of amylase secretion is coupled with alterations of beta-adrenoceptors in the rat parotid gland

Diurnal changes in the neurotransmitter receptors are important for studying the receptor function in neurophysiology. The purpose of this study is to gain an insight into the regulatory mechanisms of the diurnal variation of amylase secretion. Rat salivary amylase levels showed a diurnal variation with two peaks, a marked peak at 13 h and a lesser peak at 21 h. This increase in salivary amylase levels was completely inhibited by pretreatment of rats with the beta-adrenergic antagonist propranolol, but not by the alpha-adrenergic antagonist phentolamine. Amylase level in parotid tissue homogenate also showed a diurnal change, but there was only one peak, at 13 h. The number of maximal binding sites (Bmax) for [3H]dihydroalprenolol (DHA) in parotid membranes showed a diurnal variation with two marked peaks at 13 and 21 h, but the affinity of parotid beta-adrenoceptors for agonists or antagonists did not show any diurnal changes. Phosphorylation of nuclear non-histone proteins in the rat parotid gland showed diurnal variation with two marked peaks at 13 and 21 h. These results indicate that a diurnal variation in the number of rat parotid beta-adrenoceptors, which is presumably regulated by gene expression, is coupled with a change in salivary amylase secretion.

Effect of dietary essential fatty acid deficiency on Ca(2+)-ATPase activity in microsomal fraction of rat submandibular gland

1. The effect of dietary essential fatty acid (EFA) deficiency on Ca(2+)-ATPase activity of rat submandibular gland microsomal fraction was studied. 2. The specific activity of Ca(2+)-ATPase per milligram of microsomal protein was depressed about 35% in rats fed the EFA-deficient diet as compared with that in those fed the control diet. 3. Lineweaver-Burk plots for Ca(2+)-ATPase activity showed no significant differences in Km values for Ca2+ and ATP, but the Vmax was decreased in the EFA-deficient rats. 4. The above results suggest that depression of the Ca(2+)-ATPase activity in rats fed the EFA-deficient diet is probably due to the decrease in the Vmax of the enzyme.

Effects of 5-fluorouracil on protein synthesis and secretion of the rat parotid gland

100 mg/kg of FU were injected intraperitoneally once daily for three days. Animals were anaesthetized with 50 mg/kg of sodium pentobarbital before cannulation of the parotid duct. The total volume, amylase and protein content of the saliva were determined after stimulation with either 5 mg/kg pilocarpine or 5 mg/kg isoproterenol in FU-treated, pair-fed, and control animals. Saliva from FU-treated animals was significantly lower (p less than 0.05) in volume, amylase and protein content than that of both control groups. SDS, anionic and cationic gel electrophoresis of parotid saliva revealed no qualitative changes in the types of proteins secreted. FU reduced the total glandular amylase per unit DNA in both unstimulated and isoproterenol-stimulated parotids (p less than 0.05). Decreased protein synthesis may be the mechanism underlying the depleted secretory protein stores because the contents of isolated secretory granules from experimental glands contained less radiolabelled protein than those of either control group, and whole-gland homogenates had marked reductions in the activities of three lysosomal enzymes and in total RNA content. The secretory granules of experimental animals contained less labelled protein than those of controls, but experimental animals secreted a greater proportion of their total glandular radiolabelled secretory protein into saliva relative to amylase, suggesting that newly synthesized secretory proteins were preferentially secreted.

Similarities and differences among neuroendocrine, exocrine, and endocytic vesicles

Secretory and endocytic vesicles have analogous functions as cyclic carriers between specific cellular compartments. The centrifugally functioning secretory system operates from the Golgi complex, whereas the centripetally functioning endocytic system operates from the cell surface. Further, within polarized epithelial cells the export traffic can be directed to a distinct plasmalemmal domain which distinguishes exocrine from endocrine secretion and import traffic can be directed transcellularly. These shuttle operations involve a special class of lipid-rich, protein-poor membranes that appear to use an inwardly directed H+-translocase activity to varying extents for pH-dependent sorting and for accumulation and concentration of transported molecules. Comparative analyses of purified membrane preparations from exocrine and endocrine sources identify compositional overlap between different types of shuttle membrane. However, the structural elements that specify a particular origin or destination for a given carrier or determine function in storage and stimulus-dependent shuttling remain unknown.

Exocrine secretion granules contain peptide amidation activity

Exocrine secretion granules from the rat parotid gland contain a carboxyl-terminal peptide alpha-amidation enzyme resembling closely an enzyme from the pituitary (peptidyl-glycine alpha-amidating monooxygenase) that functions in post-translational processing of secretory polypeptides within neural and endocrine secretion granules. alpha-Amidation is a characteristic (often essential) chemical feature of a variety of biologically active regulatory peptides in animals. The parotid and pituitary activities exhibit very similar ascorbate and copper requirements, pH dependence, and kinetic properties. Further, like the pituitary enzyme(s), the parotid activity is found predominantly in secretion granule content and is discharged by exocytosis. These results establish the presence of a novel enzyme in exocrine secretion granules and suggest a potential role of the L-ascorbic acid contained in parotid granules. Two additional findings--the detection of similar levels of amidation activity in purified secretion granule fractions from other exocrine glands and the observation, in parotid granule fractions, of a B-type carboxypeptidase activity similar to that involved in post-translational processing in other systems--form a rational basis for considering whether exocrine secretion granules (like their neural and endocrine counterparts) serve as post-translational processing sites. The identity and functional role of the modified polypeptides remain to be determined.

Effect of Ni2+ on Ca2+-stimulated adenosine triphosphatase in the microsomal fraction of the rat parotid gland in vitro

Ni2+ inhibited Ca2+-stimulated adenosine triphosphatase activity in the microsomal fraction of the rat parotid gland in vitro. The Ni2+ concentration required for 50% inhibition was 0.45 mM. Inhibition mechanisms of Ni2+ for Ca2+ and ATP were of the competitive type and the noncompetitive type, respectively. The Ki values of Ni2+ for Ca2+ and ATP were 0.52 and 0.59 mM, respectively. The inhibitory effect of Ni2+ was reversible.

Correlation between Ca2+-stimulated adenosine triphosphatase activity and Ca2+ uptake in microsomal fraction of rat submandibular gland

Both the Ca2+-ATPase activity and the Ca2+ uptake in a microsomal fraction of rat submandibular gland were inhibited by the addition of indomethacin in vitro. The decrease of both the Ca2+-ATPase activity and the Ca2+ uptake caused by the drug closely paralleled each other (r = 0.97). The inhibitory manner of indomethacin on Ca2+-ATPase and Ca2+ uptake was noncompetitive for Ca2+. These results suggest that the Ca2+-ATPase in the microsomal fraction of rat submandibular gland is a Ca2+ pump in this tissue.

Effect of phospholipase A2 on Ca2+-stimulated adenosine triphosphatase activity in microsomal fraction of rat submandibular gland

The effect of phospholipase A2 on the Ca2+-ATPase (EC 3.6.1.3) activity in the microsomal fraction of rat submandibular gland was kinetically studied in vitro. The Ca2+-ATPase activity was significantly increased by the treatment with phospholipase A2 in the presence of bovine serum albumin as a scavenger for hydrolyzed products. When the microsomal fraction was incubated with phospholipase A2 in the absence of bovine serum albumin, the Ca2+-ATPase activity was not altered. The Vmax and Km values for both ATP and Ca2+ were increased by the phospholipase A2 treatment, respectively. These results indicated that the activation of Ca2+-ATPase by the phospholipase A2 treatment is due to the increase of Vmax.

Isolation and compositional analysis of secretion granules and their membrane subfraction from the rat parotid gland

A secretory granule fraction has been isolated from rat parotid by discontinuous gradient centrifugation using hyperosmotic sucrose-Ficoll solutions of low ionic strength. The secretion granule fraction comprises 25% of the total tissue alpha-amylase activity and is judged to be of high purity, both morphologically and by its low level of contamination by enzyme activities associated with other organelles. Secretion granules were lysed by capitalizing on their lability in KCl-containing media, and the low density granule membranes were separated from residual organelle and soluble contaminants by flotation in a sucrose gradient. Residual, poorly extractable secretory contaminants of the granule membrane subfraction were selectively removed by a saponin- (10 micrograms/ml) Na2SO4 (0.3 M) wash, apparently with negligible disruption of granule membrane structure. Based on detailed consideration of the extent of contamination by residual mitochondria and incompletely removed secretory polypeptides, it is possible to estimate that approximately 95% of the protein associated with the purified secretion granule membrane is bona fide granule membrane protein. Further analyses indicate that gamma-glutamyltransferase constitutes a marker enzymatic activity shared by granule membranes and the apical domain of the plasma membrane. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoretograms of radio-iodinated granule membrane polypeptides are characterized by 20-25 radioactive bands of which 5-6 are suggested to be glycoproteins by virtue of their binding of concanavalin A. The limited polypeptide composition of the secretion granule membrane (in comparison to membranes of other cellular compartments) and the high phospholipid-protein ratio (4.4 mg/mg) may reflect the functional specialization of this storage container for secretory proteins.

Distribution and regulation of mitochondrial hexokinase in rat submandibular gland

1. In rat submandibular gland, hexokinase was distributed not only in cytosol fraction but also in mitochondrial fraction. 2. Glucose-6-phosphate and ATP were most effective substances on releasing hexokinase from mitochondria. However, all the hexokinase in mitochondria could not be extracted with these substances. 3. Concentrations of glucose-6-phosphate and ATP were decreased with the administration of epinephrine in vivo. 4. Increase of the amount of mitochondria-bound hexokinase was observed for 5 min with epinephrine administration, and it returned to the control level after 10 min. 5. In rat submandibular gland, mitochondrial hexokinase may reversibly bind to and release from mitochondria as observed in brain.

Vitamin D-mediated decrease of Ca2+-pump activity in the rat parotid gland

In vitro activities of Ca2+-ATPase and 45Ca2+ uptake in microsomes, which were prepared from vitamin D-deficient rat parotid glands, were decreased in parallel by the oral administration of vitamin D3 as compared with those of control animals (r = 0.83). In vivo 45Ca2+ uptake in the parotid glands of vitamin D-deficient rats was also decreased by the oral administration of vitamin D3.

Effect of pyridoxal 5'-phosphate on Ca2+-stimulated adenosine triphosphatase activity in microsomes of rat submandibular gland in vitro

1. The Ca2+-ATPase activity in microsomes of rat submandibular gland was inhibited by pyridoxal 5'-phosphate in vitro. 2. The dissociation constant of the enzyme-pyridoxal 5'-phosphate complex was estimated to be 6.5 mM. 3. The inhibition of pyridoxal 5'-phosphate for both ATP and Ca2+ was competitive. 4. The order of inhibitory effectiveness of pyridoxal 5'-phosphate analogs was pyridoxal 5'-phosphate greater than pyridoxal HCl greater than pyridoxamine 5'-phosphate greater than pyridoxamine HCl. 5. The enzyme-pyridoxal 5'-phosphate complex was nonreducible with sodium borohydride.

Plasma membrane of the rat parotid gland: preparation and partial characterization of a fraction containing the secretory surface

A plasma membrane fraction from the rat parotid gland has been prepared by a procedure which selectively enriches for large membrane sheets. This fraction appears to have preserved several ultrastructural features of the acinar cell surface observed in situ. Regions of membrane resembling the acinar luminal border appear as compartments containing microvillar invaginations, bounded by elements of the junctional complex, and from which basolateral membranes extend beyond the junctional complex either to contact other apical compartments or to terminate as free ends. Several additional morphological features of the apical compartments suggest that they are primarily derived from the surface of acinar cells, rather than from the minority of other salivary gland cell types. Enzymatic activities characteristically associated with other cellular organelles are found at only low levels in the plasma membrane fraction. The fraction is highly enriched in two enzyme activities--K+ -dependent p-nitrophenyl phosphatase (K+ -NPPase, shown to be Na+/K+ adenosine triphosphatase; 20-fold) and gamma-glutamyl transpeptidase (GGTPase; 26-fold)--both known to mark plasma membranes in other tissues. These activities exhibit different patterns of recovery during fractionation, suggesting their distinct distributions among parotid cellular membranes. Secretion granule membranes also exhibit GGTPase, but no detectable K+ -NPPase. Since Na+/K+ adenosine triphosphatase and GGTPase, respectively, mark the basolateral and apical cellular surfaces in other epithelia, we hypothesize that these two enzymes mark distinct domains in the parotid plasmalemma, and that GGTPase, as the putative apical marker, may signify a compositional overlap between the two types of membranes which fuse during exocytosis.

Localization of amylase and mucins in the major salivary glands of the mouse

Antibodies against murine submandibular and sublingual mucins have been raised in rabbits. Both antisera appeared to be specific. Using these antibodies, the mucins were localized in the acinar cells of the submandibular and sublingual glands respectively. The dyed amylopectin method was used to estimate the activity of amylase in the salivary glands. The enzyme was localized either by a starch-substrate film method or with antibodies against purified parotid amylase. The activity of amylase in parotid homogenates is about 1000-fold higher than that in homogenates of either submandibular or sublingual glands, in which the activity was comparable. Amylase was localized in the acinar cells of the parotid gland with both localization techniques. In the sublingual gland, amylase was found predominantly in the stroma around the acini, and there was some evidence that amylase was present in the demilune cells as well. In the submandibular gland, contradictory results were obtained with both techniques. With the starch-substrate film method, amylase activity was found in the granular convoluted tubular cells, whereas immuno-reactive amylase could only be demonstrated in the acinar cells of this gland. It is concluded that in the submandibular gland amylase and mucin are present in the same cell type.

Influence of thyroid hormone on (Na+ + K+)-dependent adenosine triphosphatase activity in rat submandibular glands

Surgical thyroidectomy decreased (Na+ + K+)-dependent adenosine triphosphatase (Na-K-ATPase) activity in rat submandibular glands. Three successive doses of triiodothyronine (100 microgram/100 g of body wt) to euthyroid and hypothyroid rats produced 24 and 23 per cent increase in the enzyme activity, compared to control values. When hypothyroid rats were given smaller doses of triiodothyronine (4 microgram/100 g body wt every 48 h) for 27 days, Na-K-ATPase activity increased 40 per cent over the corresponding values in the control hypothyroid rats. Thus, thyroid hormone acts on rat submandibular glands to increase the Na-K-ATPase activity of that target tissue.

ATP-induced lysis of rat parotid secretory granules: possible role of ATP in exocytotic release

Secretory vesicles isolated from a variety of mammalian tissues are known to lyse and thereby release their secretory products when exposed to ATP. This process, which will be termed ATP-induced lysis, has been studied most extensively using adrenal chromaffin-granule preparations. We report here that ATP causes the lysis of a highly purified preparation of rat parotid secretory granules. The rate of granule lysis was measured spectrophotometrically, and ATP-induced lysis was expressed as the increase in the rate of lysis (r = % lysis per min) when ATP was added. This lytic process was characterized with respect to pH, temperature, osmolarity, and the ionic composition of the media. ATP-induced lysis of parotid granules was found to have the following properties in common with the extensively characterized chromaffin-granule process: 1. It is a saturable function of ATP with half-maximal rates observed at 0.5 +/- 0.1 mM ATP. 2. It is temperature dependent, eg, r = 6.1 +/- 2.1%/min at 30 degrees C vs 12.2 +/- 2.5%/min at 37 degrees C. 3. It is inhibited in hyperosmotic media, eg, r = 5.3 +/- 0.3%/min at 0.3 OsM vs 0.8 +/- 0.2%/min at 0.4 OsM. 4. It shows a nucleotide preference of ATP = GTP greater than ADP greater than AMP greater than CTP = ITP. 5. It has an anion requirement. The above findings, combined with reports of ATP-induced lysis of cholinergic, insulin, and posterior-pituitary vesicles, imply that ATP-induced lysis may reflect an ATP-dependent property of all secretory vesicles, and as such, this vesicle property could play a similar role in each exocytotic release process. Using a model system, Miller and Racker [22] made a surprising finding that the extent of which liposomes fuse with a black lipid membrane depends on the osmotic gradient across the vesicle membrane. In view of the osmotic dependence of ATP-induced lysis in this and other secretory-vesicle preparations, we postulate that ATP may prime secretory vesicles for fusion with the plasma membrane by inducing and/or maintaining an osmotic gradient across the vesicle membrane.

Effect of indomethacin on Ca2+-stimulated adenosine triphosphatase in the microsomes of rat submandibular gland

The effect of indomethacin, which is a nonsteroidal anti-inflammatory drug, on Ca2+-stimulated adenosine triphosphatase (Ca2+-ATPase) activity in the microsomes of rat submandibular gland was investigated. The effect of aspirin on Ca2+-ATPase activity was also studied.

Diphenylhydantoin effects on salivary secretion and microsomal calcium accumulation and release

Diphenylhydantoin inhibited calcium uptake into dog submandibular microsomes at concentrations of 0.05 mM and greater but did not affect calcium release; mesantoin and hydroxyphenyl-phenylhydantoin were without effect. Diphenylhydantoin (20 mg/kg) given i.v. significantly reduced, and mesantoin slightly reduced secretion from submandibular and parotid glands, but hydroxyphenyl-phenylhydantoin was without effect. These data suggest that the reduced secretory volume produced by diphenylhydantoin may be related to its effect on transmembrane calcium movements.

Studies on intracellular transport of secretory proteins in the rat exocrine pancreas. III. Effect of cobalt, lanthanum and antimycin A

The effects of cobalt and lanthanum on the secretory process of the rat exocrine pancreas was studied in vitro using isolated pancreatic lobules. Cobalt in concentrations between 10(-3) to 10(-5) M has no effect on the rate of protein synthesis, intracellular transport, or discharge of zymogen granules, if the total population of stored granules is considered. It has, however, a marked effect on the release of newly packed zymogen granules which are formed during incubation in 10(-3) M CoC1(2). Determination of specific radioactivity in amylase released under the stimulation of 5X10(-6) M carbamylcholine and of total proteins retained in the zymogen granule fraction during stimulation indicate that granules formed during incubation in CoC1(2) are excluded from discharge. Lanthanum, on the other hand, has a differential effect on protein synthesis, intracellular transport, and discharge. Incorporation of tritiated leucine into TCA-precipitable proteins is inhibited by 50% at 10(-3) M LaC1(3). Intracellular transport as studied by cell fractionation is not changed during the first 35 min post pulse but is delayed from then on. This late effect is more pronounced if pancreatic lobules are preincubated for 60 min in 10(-3) M LaC1(3). Discharge of amylase and newly synthesized proteins is inhibited dose-dependently up to 80% by 10(-3) M LaC1(3). The effects of both cobalt and lanthanum are not due to an inhibition of cellular respiration. Comparison of these results with the inhibitory action of antimycin A between 10(-4) to 10(-8) M concentrations reveals a dose-dependent diminution of the rate of protein synthesis and intracellular transport, while discharge of granules is less energy dependent. The fine structural appearance of pancreatic lobules after 3 hrs incubation in 10(-3) M CoC1(2) is not altered, while in 5X10(-3) and 10(-3) M lanthanum acinar lumina are enlarged and the apical cytoplasm contains large vacuoles. At the highest concentration of lanthanum a flocculent electron dense material is observed apposed to the external lamina of the plasma membrane. The distribution of this material on the membrane is described. Antimycin A leads to cellular changes corresponding to the irreversible inhibition of cellular respiration. It is concluded from the results that cobalt acts on the process of granule formation inside the cell, while lanthanum by its binding to the plasma membrane may alter molecules involved in secretagogue binding and transport systems into the cell.

Studies on intracellular transport of secretory proteins in the rat exocrine pancreas. II. Inhibition of antimicrotubular agents

The possible role of microtubules and microfilaments in the secretory process of the rat exocrine pancreas was analysed in vitro using isolated pancreatic lobules. Colchicine and vinblastine as microtubule inhibitors, hexylene glycol as a microtubule stabilizer, and cytochalasin B as a disruptive agent for microfilaments were used in increasing concentrations to test their effects on protein synthesis, intracellular transport, zymogen discharge, and cellular respiration. Colchicine only at 10(-2) M concentrations inhibits protein synthesis, while vinblastine inhibits at 10(-6) and 10(-5) M by 20% and at 10(-4) M by 55%. A similar inhibition is observed with 1.5% concentrations of hexylene glycol while cytochalasine B at 1,5 and 10 mug/ml is without effect on protein synthesis. Colchicine and vinblastine have their major effects on intracellular transport both in secretion studies and cell fractionation experiments. Colchicine in concentrations between 10(-3) to 10(-5) M inhibits discharge of newly synthesized proteins by 50%, while vinblastine shows a dose-response relationship of 40% inhibition of 10(-6) M to 90% at 10(-4) M. Discharge of amylase is uniformly reduced by 30% by both colchicine and vinblastine in the whole dose range. The pronounced effect of colchicine and vinblastine is evident in cell fractionation studies: both drugs inhibit the disappearance of protein radioactivity from microsomes and its appearance in zymogen granules; similarly the peak radioactivity in smooth microsomes (Golgi) appears delayed. No differential effect on the secretory process was observed with 1.5% concentrations of hexylene glycol or cytochalasin B at 1.5 and 10 mug/ml concentrations. A fines tructural analysis of microtubules and microfilaments in the exocrine pancreatic cell reveals their distribution in all parts of the cytoplasm and in relation to all cell organelles. Both systems (microtubules, microfilaments) seem to be connected, at least in certain areas of the cytoplasm and at the plasma membrane. The reduction of transport efficiency by microtubule inhibitors results in a deposition of secretory material in the cristernal space of the rough endoplasmatic reticulum, which leads to the formation of paracrystals. Colchicine at 10(-3) M concentrations leads to an enlargement of condensing vacuoles in the Golgi complex.

Studies on intracellular transport in the rat exocrine pancreas. I. Inhibition by aromatic amino acids in vitro

In vitro incubation of rat pancreatic lobules in the presence of 10 mM concentrations of 2 natural (phenylalanine, tryptophane) and 2 modified aromatic amino acids (p-fluorophenylalanine, p-chlorophenylalanine) induces paracrystal formation in the cisternal space of the rough endoplasmic reticulum and in the acinar lumen. Aggregation of secretory material in transitional elements of the rough endoplasmic reticulum suggests tubular connection to the Golgi complex. Paracrystal formation is correlated with a disturbance of the three major phases in the secretory process of the exocrine cell. Incorporation of radioactive amino acids into proteins is inhibited by 10 mM concentrations of phenylalanine and tryptophane by 20 and 50% respectively and by p-chlorophenylalanine at 1 and 10 mM concentrations by 50 and 75%. The inhibition of protein synthesis is not due to a reduced intracellular concentration of radioactive precursor amino acids. Intracellular transport of newly synthesized proteins as studied by a radioassay for zymogen discharge and by cell fractionation is similarly inhibited by phenylalanine, tryptophane and p-chlorophenylalanine at 10 mM concentrarions (20, 30, and 40% respectively). Discharge of zymogens as measured by the secretion of amylase stimulated with 5 X 10(-6) M carbamylcholine is reduced by 20% if 10 mM concentrations of phenylalanine, tryptophane or p-chlorophenylalanine are present in the medium. Paracrystals were isolated by differential centrifugation and their protein content compared with isolated zymogen granules. On sodium dodecylsulfate gel electrophoresis paracrystalline proteins show the same electrophoretic pattern as the content of zymogen granules.

Purification of zymogen granules from monkey parotid glands

A method giving highly purified zymogen granules from Macaca irus and Cercopithecus aethiops parotid glands in reported. A 0.3 M sucrose medium for homogenization was supplemented with 10 mM tris/HCl, pH 7.3, and 0.1 mM lauric acid to stabilize the fragile monkey zymogen granules. Nuclei and cell debris were sedimented at 150 times g. A "crude" zymogen granule fraction was trapped in the 1.0 M sucrose layer of a discontinuous sucrose gradient at 1000 times g. Equilibrium centrifugation in a continuous sucrose gradient gave a fraction of zymogen granules at 1.85 M sucrose. Compared to the homogenate, this fraction exhibited about two-fold increase in zymogen granule marker, whereas mitochondrial marker was reduced to 1/4 and lysosomal marker to 1/2. A low level of contamination from other cell organelles was confirmed by electron microscopic investigation.

The proteins of the content of the secretory granules of the rat parotid gland

The proteins of the secretory granules of the rat parotid gland were characterized by sodium dodecylsulfate gel electrophoresis, by chromatography of [3-H]proline-labeled proteins on DEAE-cellulose and by amino acid analysis. Sodium dodecylsulfate gel electrophoresis of the secretory granule content showed five principal proteins and a limited number of minor components. Only two of the principal bands could be identified as known secretory enzymes of the parotid gland. One was identified as the alpha-amylase and one as deoxyribonuclease. Peroxidase and ribonuclease form minor portions of the secretory proteins. The other three major proteins constitute, together, about 60% by weight, of the secretory granule content proteins. Of these, one which represents more than 30% of the total granule protein was found to contain uniquely high amounts of leucine residues (21 mole%). Another one of these principal proteins was relatively rich in cysteine residues (7 mole%). The fifth principal protein was found to contain high amounts of proline (28 mole%) glutamic acid (17 mole%) and glycine (18 mole%) residues. Its amino acid composition was very similar to that of the proline-se granules. This protein, however, differed from the "membranous" proline-rich proteins by several criteria. Two minor glycoproteins of the secretory granule content were also found to be rich in proline residues (37 mole%). As with the other proline-rich proteins of the granule, they contained no sulphur-containing amino acids, stained faintly pink with Coomassie Blue and were underestimated by the Lowry method. They differ however, from all the other proline-rich proteins of the granule by having a significantly higher content of threonine, less glycine (9 mole%) and much less glutamic acid (3 mole%). Of the principal proteins, only the deoxyribonuclease and the half-cystine-rich proteins were positively stained by periodic acid Schiff staining. The possible functions of the leucine-rich, the half cystine-rich and the various proline-rich proteins are discussed.

Studies on Ca2+-stimulated adenosine triphosphatase in rat submandibular glands: its distribution and properties

Intracellular distribution and some characteristics of a Ca2+-stimulated adenosine triphosphatase (ATPase) in rat submandibular glands were investigated. This enzyme was activated by calcium alone, and magnesium was not necessary for its activation. Mg2+-stimulated ATPase also was investigated in the same enzyme preparations.

Radioautographic analysis of the secretory process in the parotid acinar cell of the rabbit

Intracellular transport of secretory proteins has been studied in the parotid to examine this process in an exocrine gland other than the pancreas and to explore a possible source of less degraded membranes than obtainable from the latter gland. Rabbit parotids were chosen on the basis of size (2-2.5 g per animal), ease of surgical removal, and amylase concentration. Sites of synthesis, rates of intracellular transport, and sites of packaging and storage of newly synthesized secretory proteins were determined radioautographically by using an in vitro system of dissected lobules capable of linear amino acid incorporation for 10 hr with satisfactory preservation of cellular fine structure. Adequate fixation of the tissue with minimal binding of unincorporated labeled amino acids was obtained by using 10% formaldehyde-0.175 M phosphate buffer (pH 7.2) as primary fixative. Pulse labeling with leucine-(3)H, followed by a chase incubation, showed that the label is initially located (chase: 1-6 min) over the rough endoplasmic reticulum (RER) and subsequently moves as a wave through the Golgi complex (chase: 16-36 min), condensing vacuoles (chase: 36-56 min), immature granules (chase: 56-116 min), and finally mature storage granules (chase: 116-356 min). Distinguishing features of the parotid transport apparatus are: low frequency of RER-Golgi transitional elements, close association of condensing vacuoles with the exit side of Golgi stacks, and recognizable immature secretory granules. Intracelular processing of secretory proteins is similar to that already found in the pancreas, except that the rate is slower and the storage is more prolonged.