Phosphofructokinase from the epithelial cells of rat small intestine. Comparison of regulatory properties with those of skeletal muscle, liver and brain phosphofructokinase

The regulatory properties of phosphofructokinase from rat mucosa, liver, brain and muscle were investigated. Mucosal phosphofructokinase displayed cooperativity with respect to fructose 6-phosphate at pH 7.0 and so did the muscle, brain and liver isoenzymes. All these four isoenzymes were inhibited by ATP, the mucosal isoenzyme being the least inhibited. They were also inhibited by citrate and creatine phosphate. AMP, ADP, glucose 1,6-diphosphate, fructose 2,6-bisphosphate and inorganic phosphate were all strong activators for the mucosal, brain, liver and muscle phosphofructokinase, but the mucosal isoenzyme was found to be more activated than the others, accounting for the higher rates of glycolysis observed in mucosa. The results suggest that mucosal phosphofructokinase is unique and different from all the other isoenzymes.     

The isolation and characterization of phosphofructokinase from the epithelial cells of rat small intestine.

1. Only a single phosphofructokinase isoenzyme is present in the mucosa of rat small intestine. 2. Mucosal phosphofructokinase was purified to yield a homogeneous preparation of specific activity 175 units/mg of protein. 3. The native enzyme is a tetramer, with monomer Mr 84 500 +/- 5000. 4. The native enzyme may be degraded by the action of endogenous proteinases to give two products with the same specific activity as the native enzyme: degradation occurs in the order native enzyme leads to proteolytic product 1 leads to proteolytic product 2. 5. Proteolytic product 1 has a greater mobility in cellulose acetate electrophoresis at pH8 and binds more strongly to DEAE-cellulose than does native enzyme; the converse is true for proteolytic product 2. 6. Proteolytic product 1 is a tetramer with a monomer Mr about 74 300; proteolytic product 2 is also a tetramer. 7. Native enzyme can only be prepared in the presence of proteinase inhibitors; partial purifications based on simple fractionation of crude mucosal extracts in the absence of proteinases inhibitors contain proteolytic product 2 as the main component and proteolytic product 1 together with little native enzyme. 8. Purified native mucosal phosphofructokinase displayed little co-operativity with respect to fructose 6-phosphate at pH 7.0 and was only weakly inhibited by ATP.     

Phosphodiesterase II in epithelial cells from guinea-pig and rat small intestine

Phosphodiesterase II activity was determined by using a synthetic substrate, the 2,4-dinitrophenyl ester of thymidine 3'-phosphate. The enzyme activity was determined in fractions obtained by differential centrifugation of homogenates of epithelial cells from the small intestinal mucosa of guinea pigs and rats. In guinea-pig preparations phosphodiesterase II occurred with highest specific activity in those fractions rich in succinate dehydrogenase and acid phosphatase. A lysosomal location for the guinea-pig enzyme was indicated by its structure-linked latency and by its association with particles that under-went a characteristic decrease in equilibrium density when Triton WR-1339 was injected into the animals. With rat preparations a much greater proportion of the phosphodiesterase II activity was found in the soluble fraction after ultracentrifugation. The rat enzyme exhibited a lower degree of latency and administration of Triton WR-1339 had no effect. The rat enzyme further differed from that of the guinea pig in other respects; it was more labile at 60 degrees C, it exhibited a lower pH optimum and it had a higher molecular weight as determined by gel-filtration chromatography.     

Lipoproteins of rat small intestine epithelial cells in D-hypovitaminosis

Chemical composition of lipoproteins has been studied in intestinal epitheliocytes of rats in normalcy and under D-hypovitaminosis. It is found that D-hypovitaminosis induces changes in the lipid and protein composition of lipoproteins. It is supposed that disturbances in biosynthesis of the lipoprotein components and their transport may be possible reasons of such changes.     

Allosteric properties of phosphofructokinase from the epithelial cells of thermally injured rat small intestine

1. The allosteric properties of phosphofructokinase from the epithelial cells of thermally injured rat small intestine were studied and compared with those properties of the normal rats. 2. The fructose 6-phosphate saturation curve of mucosal phosphofructokinase from thermally injured rats (3 days post injury, 33% of body surface area) displayed cooperatively; the ratio of the activity observed at pH 7.0 in the presence of 0.5 mM fructose 6-phosphate and 2.5 mM-ATP to the optimal activity at pH 8.0, v 0.5/V, was 0.42 +/- 0.02 in the normal rats and 0.22 +/- 0.03 in the injured rats. 3. The enzyme from thermally injured rats was very sensitive to inhibition by ATP as compared to that from normal rats. 4. The enzyme from thermally injured rats was inhibited by citrate and phosphocreatine in a synergistic manner with ATP. 5. Activation under nearly cellular conditions was produced by ADP, AMP and glucose-1,6-biphosphate. 6. In general, the mucosal enzyme of thermally injured rats was more susceptible to inhibition or activation by various metabolites than the enzyme of the normal rats. 7. These results may suggest that mucosal phosphofructokinase of thermally injured rats may not be subject to the same control mechanism as the normal rats in vivo due to changes in the concentrations of fructose-2,6-biphosphate.     

Protein kinase C from small intestine epithelial cells

Protein kinase C activity has been identified in cytosolic and membrane fractions from rat and rabbit small intestine epithelial cells. The cytosolic fraction comprised about the 75% of total activity. Protein kinase C activity was resolved from other protein kinase activities by ion exchange chromatography. Phosphatidylserine or phosphatidylinositol were required for protein kinase C to be active. In addition, the activity was enhanced by the presence of a diacylglycerol. Diolein and dimyristin were the most effective (13-14 fold activation). In the presence of phosphatidylserine and diolein, the Ka for activation by Ca2+ was 10(-7)M. The phorbol ester TPA substituted for diacylglycerol in activating protein kinase C. Brush border and basolateral membranes contained protein kinase C activity, although the specific activity of the basal lateral membranes was four-fold higher than the specific activity of the brush border membranes. The presence of PKC in small intestine epithelial cells might have important implications in the Ca2+ mediated control of ionic transport in this tissue.     

Some characteristics of monocarboxylic acid transfer across the cell membrane of epithelial cells from rat small intestine.

1. The translocation of monovalent organic anions (pyruvate, propionate, acetate and butyrate) across the cell membrane of isolated epithelial cells from rat small intestine was studied by measuring competitive inhibition kinetics, exchange diffusion and temperature dependence of the efflux rate. A possible function of a monocarboxylate carrier in intestine will be discussed. 2. Earlier studies on the inhibition of pyruvate transport of fatty acids were extended to propionate and found to show the same characteristics. The kinetics, however, appeared to be more complex by the contribution of several diffusion pathways for propionate. 3. The mechanism of countertransport was most compatible with an "accelerated exchange diffusion" and could be studied at both sides of the membrane. This exchange diffusion exhibited saturation kinetics. It is proposed that different monocarboxylate anions may have different affinities for a common carrier. 4. Temperature dependence of the efflux of pyruvate and propionate was studied. Arrhenius plots obtained were not found to be linear between 0 and 5 degrees C. Between 5 and 15 degrees C activation energies for pyruvate and propionate efflux rates were found to be 19.6 and 12.6 kcal/mol, respectively.     

α-Adrenergic inhibition of cyclic AMP accumulation in epithelial cells isolated from rat small intestine

The present work shows that α-adrenergic agonists induce the suppression of basal and hormone-stimulated cyclic AMP levels in rat intestinal epithelial cells. Epinephrine (100 μM) suppresses by 35% the cyclic AMP levels evoked by the vasoactive intestinal peptide (VIP). The adrenergic agent induces a similar percentage of inhibition at 15, 30 and 37°C. Addition of epinephrine 20 min prior to, on 5 or 20 min after VIP yields the same magnitude of inhibition as when performed together with the stimulus. The α-adrenergic agent does not alter the K_0.5 of VIP in stimulating cyclic AMP production but reduces its efficacy. Epinephrine also suppresses prostaglandin E₁- and E₂-stimulated cyclic AMP levels by about 35%. The lowest effective concentration of epinephrine required to suppress VIP-stimulated cyclic AMP levels is 0.1 μM, half-maximal (K_0.5) and maximal effects being observed at 5 and 100 μM, respectively. Norepinephrine has the same efficacy but a slightly lower potency (K_0.5 = 18 μM) than epinephrine. Phenylephrine acts as a partial agonist of very low potency; clonidine has very little intrinsic activity and antagonizes the inhibition by epinephrine. The inhibition of VIP-stimulated cyclic AMP levels is observed in the absence of any blocking agents. It is not affected by the β blocker propranolol, but is completely reversed with α blockers with the following order of potency: dihydroergotamine>yohimbine>phentolamine. Yohimbine is much more potent than prazosin, which only partially reverses the inhibition induced by epinephrine. It is concluded that α-adrenoreceptors of the α₂ subtype mediate the suppression of VIP-stimulated cyclic AMP levels in intestinal epithelial cells. This effect is likely to be due to the inhibition of adenylate cyclase within intact cells as epinephrine is able to reduce adenylate cyclase activity of intestinal epithelial cell plasma membranes.     

Quantitative changes in alkaline phosphatase in epithelial cells of rat small intestine from birth to weaning

Synopsis Alkaline phosphatase activity has been measured in mucosal scrapings of rat small intestine from birth to weaning. The levels of activity were then compared with those found in brush border fractions and cell residues over the same period. The amount of activity found in the residues was too high and variable to be accounted for in terms of diffusion, and it was concluded that during the developmental period considerable quantities of enzyme were present in the cytoplasm.The duodenal changes were similar, in form, in both mucosal scrapings and brush border, but the changes in the ileal brush borders were markedly different from those found in the ileal mucosal scrapings. A peak in the activity measured in the brush border fraction was observed during the weaning period. A partial explanation for the observed changes is suggested but a detailed analysis was not possible at the present stage in the work.     

N-linked glycopeptides with blood group determinants lacking neuraminic acid from the epithelial cells of rat small and large intestine.

The N-linked type of glycans were prepared as their glycopeptides after pronase digestion of the epithelial cells from the small and large intestine of two inbred strains of rat. These glycopeptides were analysed for sugar composition, for blood-group activity, by 1H-NMR spectroscopy, and after permethylation by electron-impact mass spectrometry. The glycopeptides were of the triantennary and tetraantennary types with intersected GlcNAc. The terminal parts were, in contrast to most N-linked glycans, devoid of neuraminic acid residues. Instead they contained blood-group determinants. Blood-group-H types 1 (Fuc alpha 1-2Gal beta 1-3GlcNAc) and 2(Fuc alpha 1-2Gal beta 1-4GlcNAc) were found in the small and large intestines of both strains, although type-1 predominated. One rat strain (GOT-W) did not express blood-group-A glycopeptides in the small intestine, but the large intestine from the same strain did. The other strain (GOT-BW) expressed blood-group-A determinants in the small intestine. The lack of neuraminic acid residues in the small and large intestine and of blood-group-B activity in the large intestine differed from that found in glycosphingolipids obtained from the same organs.     

Regulation of 6-phosphofructo-1-kinase from the epithelial cells of rat small intestine during pregnancy and lactation

The regulation of 6-phosphofructo-1-kinase (PFK) in the epithelial cells of rat small intestine was studied during pregnancy and lactation. The total activities and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (nu 0.5/V] of the partially purified mucosal PFK were found to increase initially in early pregnant rats (11-12 days of gestation) and to fall back to normal in late pregnant rats (19-20 days of gestation). These changes in enzyme activity during pregnancy were associated with similar changes in the circulating levels of progesterone. The maximal activity and activity ratio (nu 0.5/V) were increased in male and female rats injected with progesterone. An increase in the total activity and activity ratio of mucosal PFK was also obtained in lactating rats. However, the enzyme was not strongly activated by inorganic phosphate, fructose 2,6-bisphosphate or glucose 1,6-bisphosphate either in early pregnant or lactating rats. These results indicate that mucosal PFK is already present as an active form during early pregnancy and lactation. Therefore, it is suggested that female sex hormones increase the circulating levels of insulin during early pregnancy which, in turn, positively affect the activity of mucosal PFK which could be also stimulated by the increased levels of fructose 2,6-bisphosphate. The increased activity of PFK in the peak lactating rats could be possible because of an increased demand for lactate production from glucose together with the stimulation of PFK by the increased concentrations of fructose 2,6-bisphosphate which therefore increases the rate of glycolysis.     

Close relationships between the cells of the immune system and the epithelial cells in the rat small intestine

Summary A possible contribution of the intestinal epithelium to the immune defence system was studied by electron microscopy in the rat small intestine. The cells of the immune system (CIS) such as lymphocytes, eosinophils and macrophages penetrate the basal lamina into the epithelium and make close relationships with the absorptive cells. At the points of close apposition, the two cell membranes run parallel at a regular distance of about 20 nm. On the other hand, about 5% of the intestinal absorptive cells also penetrate the basal lamina into the lamina propria via their basal protrusions and show a similar close association with CIS. The basal protrusions contain many microfilaments; this indicates that they are structures with a definite function rather than a simple hernia. These findings are discussed with respect to the transport of antigenic molecules and of intercellular communication between CIS and the intestinal epithelium.     

Characterization of immunoreactive motilin from the rat small intestine.

Immunocytochemistry, radioimmunoassay, chromatography, and biological assay using a rabbit isolated duodenal muscle strip preparation were used in attempting to characterize motilin from the rat small intestine. Several different antisera and monoclonal antibodies directed against natural porcine motilin were used. A variety of fixation techniques using Bouin's, paraformaldehyde, and benzoquinone with different staining methods including, fluorescein-conjugated second antibody, peroxidase-antiperoxidase or peroxidase-conjugated second antibody techniques were used. All methods failed to detect immunoreactive motilin cells in the rat small intestine. The same antisera were used in radioimmunoassays for motilin to evaluate extracts of rat intestinal tissue. Two of these detected immunoreactive motilin in gut extracts, and these antisera showed a different distribution for the peptide. Samples containing immunoreactive motilin obtained from cation exchange chromatography on SP-Sephadex-G25 were concentrated and assayed for biological activity in a rabbit duodenal muscle strip preparation. Desensitization of duodenal tissue to porcine motilin could be demonstrated by pretreatment with this peptide. The biological activity of partially purified rat intestinal immunoreactive motilin was not prevented by pretreatment of the tissue with motilin. Further purification of this preparation on Bio-Gel P-10 yielded an immunoreactive motilin peak that co-eluted with natural porcine motilin. Rat intestinal immunoreactive motilin did not co-elute with natural porcine motilin following high pressure liquid chromatography on a Waters microBondapak C18 reversed-phase column using a linear gradient of water-acetonitrile (10-45%) over 30 min. Although of similar molecular size, rat motilin is probably structurally dissimilar to other mammalian motilins.     

The effect of starvation on the control of phosphofructokinase activity in the epithelial cells of the rat small intestine.

1. The effect of depriving rats of food for 48 h on the specific activity of phosphofructokinase in the epithelial cells of the small intestine and on the regulatory properties of the enzyme displayed in crude (particle-free) mucosal extracts was studied. 2. The specific activity of phosphofructokinase, measured under optimal conditions at pH8, in the mucosa of fed rats showed a negative aboral gradient along the intestine, decreasing from 15.2 +/- 1.2 units (mumol/min)/g wet wt. in the proximal jejunum to 4.6 +/- 1.2 units/g wet wt. in the terminal ileum. 3. After starvation, the gradient was diminished, but not abolished; the diminution in gradient was due almost exclusively to a decrease in the specific activity of phosphofructokinase in the proximal jejunum by about 30%, there being no change in the terminal ileum. 4. In fed rats, the susceptibility of phosphofructokinase to inhibition by ATP, when assayed in crude mucosal extracts under suboptimal conditions, was independent of length along the small intestine; the ratio of the activity observed at pH 7.0 in the presence of 0.5 mM-fructose 6-phosphate and 2.5 mM-ATP to the optimal activity at pH 8, v0.5/V, was 0.36 +/- 0.05 in the proximal jejunum and 0.42 +/- 0.07 in the terminal ileum. 5. After starvation, the susceptibility of phosphofructokinase to inhibition by ATP was increased and was again found to be independent of length along the small intestine: after starvation, v0.5/V was 0.19 +/- 0.04 and 0.20 +/- 0.07 for the proximal jejunum and the terminal ileum respectively. 6. Re-feeding of previously starved rats on a high-carbohydrate diet overnight for 16 h restored both the specific activities of phosphofructokinase and its susceptibility to inhibition by ATP to normal values for fed rats. 7. The data support the idea that the specific activities and the regulatory properties of phosphofructokinase in the epithelial cells of rat small intestine are mediated by distinct humoral factors. 8. The changes in glucose utilization rate of the jejunum when rats are starved can in principle be accounted for by a combination of changes in the specific activity and in the regulatory properties of mucosal phosphofructokinase.