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D S Parsons

Publications and source records attributed to D S Parsons.

At least 109 records · Page 6Linked to original sources
PubMed1976

The role of some small peptides in the transfer of amino nitrogen across the wall of vascularly perfused intestine.

The characteristics have been investigated of the transfer into the vascular bed of L-leucine and glycine from free amino acids or peptides in the intestinal lumen of Rana pipiens. Over the concentration range 0-5-10 mM the transfer of L-leucine is but little affected by the presence of equimolar concentrations of glycine but the transfer of glycine, in contrast, is greatly inhibited by the presence of L-leucine. 2. With glycyl-L-leucine in the intestinal lumen, the rate of transfer of glycine into the vascular bed is much greater than from the mixture of the two amino acids and is equal to that of the L-leucine. From L-leucyl-glycine the rates of transfer of leucine and of glycine are also higher than from the mixture of the two amino acids but the rate of transfer of glycine is somewhat lower than that of leucine. There is no evidence of the presence of the dipeptides in the effluent from the portal vein. 3. When the peptide glycyl-L-leucine is added to the lumen in the presence of 10 mM concentrations of the free amino acids, additional amounts of L-leucine and of glycine are transferred in approximately equimolar quantities into the vascular bed. This additional transfer exhibits saturation with respect to concentration of peptide in the intestinal lumen. An additional transfer of amino acids was also found when L-leucyl-glycine was added to the lumen in the presence of saturating concentrations of the two amino acids. 4. Evidence is presented that the presence of the dipeptides in the intestinal lumen had little effect on the transfer of free amino acids from the lumen into the vascular bed. Although the transfer of free amino acids from the lumen into the vascular bed is significantly, but not completely, abolished when the Na in the intestinal lumen is replaced by K, the transfer of the amino acids from the dipeptides is but little affected. 5. The findings are discussed in relation to the view that the dipeptides are transported into the mucosal epithelium by a process that is distinct from those promoting uptake of the individual amino acids. Being completely hydrolysed, there is no evidence for an accumulative uptake of the peptides; it is suggested that this may be related to the fact that the peptide uptake occurs in the absence of Na in the intestinal lumen.

Animals↗
PubMed1975

Amino acid movements across the wall of anuran small intestine perfused through the vascular bed.

1. L-leucine transfer across the wall of the small intestine has been studied in a vascularly perfused preparation from four species of frog. Some properties of the preparation are described. 2. A description is given of the endogenous amino acids appearing in the vascular bed and of the kinetic properties of this washout of endogenous L-leucine. 3. In the steady state of absorption, the transfer function relating the net flux of exogenous L-leucine into the vascular bed to the concentration in the lumen exhibits saturation. Under the conditions of the experiments the apparent concentration in the lumen for half-maximum transfer of L-leucine is found to be 2-1 +/- 0-4 (5) mM. 4. When Na ions are removed from the lumen the transfer of L-leucine into the vascular bed is inhibited. However, the additional removal of Na ions from the fluid in the vascular bed is further inhibitory to the transfer of the amino acid. 5. L-leucine previously absorbed from the lumen appears in the vascular bed in a biphasic fashion. Estimates are deduced of the size of the pool of L-leucine within the tissue which drains into the vascular bed. 6. These results are discussed in relation to previous work on amino acid transport undertaken with various sorts of preparation of small intestine.

Albumins↗
PubMed1974

The transamination of glutamate and aspartate during absorption in vitro by small intestine of chicken, guinea-pig and rat.

1. Procedures are described for direct measurement of the extent and rate of transamination of glutamate and aspartate over periods of up to 90 min, during absorption in vitro by the small intestine of chicken, guinea-pig, and rat.2. During absorption of dicarboxylic amino acids by rat small intestinal segments circulated through the lumen in vitro, alanine contributed up to 85% of the amino acids appearing in the fluid secreted at the serosal surface. In guinea-pig and chicken intestine, the proportion of alanine in the secreted amino acids did not exceed 60%.3. For the different species studied, a relationship was found between the extent to which the dicarboxylic amino acids were transaminated to alanine and the total amount of GPT found in other studies to be present in the intestinal mucosa. In both rat and guinea-pig small intestine, the proportion of alanine in the total amino acids appearing at the serosal surface was similar in the jejunum and ileum. The rate of appearance of alanine in serosal fluid was greater in the ileum than in the jejunum of the rat.4. Reasons are given for supposing that for all the species studied there is a limit to the capacity of the small intestinal mucosa to subject free dicarboxylic amino acids to transamination. It is concluded, however, that it is unlikely that this capacity will be exceeded under in vivo conditions.

Alanine↗
PubMed1972

The isoenzymes of carbonic anhydrase: kinetic properties with particular reference to the functions in the intestinal tract.

1. Details are given of an electrometric method for measuring the activity of isoenzymes of carbonic anhydrase (EC 4.2.1.1) in catalysing the hydration of carbon dioxide under different conditions at 0 degrees C. In the method, a measured volume of water saturated with carbon dioxide at a known partial pressure and appropriate temperature is introduced into a buffered solution. Using a sensitive electrometer and recording instrument, the subsequent change in hydrogen ion concentration is recorded as a function of time. Under the conditions of assay, the pH change induced in the presence of substrate is very small (DeltapH < 0.05 units) and the period of observation need not exceed 10 sec.2. For enzymes isolated from guinea-pig tissues, it is found that the specific activity of the ;high activity' isoenzyme (carbonic anhydrase C, carbonic anhydrase II, HACA) is about eighteen times that of the ;low activity' counterpart (carbonic anhydrase B, carbonic anhydrase I, LACA) when measured at 0 degrees C, pH 7.2, and ionic strength 0.19. Under the same conditions, the K(m) was found to be 10 mM for the ;high activity' isoenzyme and 23 mM for the ;low activity' isoenzyme. No differences were found between the equivalent kinetic parameters of the corresponding isoenzymes isolated from different tissues.3. The isoenzymes isolated from guinea-pig tissues are found to be inhibited by acetazolamide in a non-competitive manner. It is also found that the ;high activity' isoenzyme is many times more sensitive to this inhibitor than is the ;low activity' isoenzyme. Evidence is presented which indicates that one acetazolamide binding site is present on each molecule of either isoenzyme.4. While chloride ions specifically inhibit the ;low activity' component of guinea-pig carbonic anhydrase (I(0.5) = 40 mM), acetate, butyrate and pyruvate inhibit both isoenzymes. Under the conditions employed, acetate and pyruvate are more strongly inhibitory to the ;low activity' isoenzyme than to the ;high activity' isoenzyme, while butyrate is more strongly inhibitory to the ;high activity' isoenzyme.5. The findings are discussed with particular reference to the physiological significance of the presence of the isoenzymes in the gastro-intestinal tract. Also considered are possible relationships between the distribution of the ;low activity' isoenzyme in these tissues and the transport and metabolism of products of fermentation occurring in the intestinal lumen.

Acetates↗
PubMed1972

Oligopeptidases of brush border membranes of rat small intestinal mucosal cells.

1. The properties of peptidases located in the brush borders of rat small intestinal mucosal cells have been investigated using a new method for the assay of peptidase activity. In this method amino acids produced by hydrolysis of peptides are oxidized by ophidian L-amino acid oxidase and the hydrogen peroxide produced during reoxidation of the reduced enzyme is estimated spectrophotometrically.2. 10-20% of the total cellular peptidase activity and 50% of the leucylnaphthylamidase (LNAase) activity were found to be tightly bound to the brush borders and to possess different substrate specificity from the supernatant peptidase activity.3. Evidence from kinetic and competition studies indicates the presence of more than one peptidase in the brush borders. The peptidases exhibit pH optima of 8.0-8.5, are inhibited by EDTA, but are not usually activated by divalent metal ions. The brush border peptidases hydrolyse di- and tripeptides, some oligopeptides, leucinamide and leucyl-beta-naphthylamine. On the basis of the Michaelis constants, these substrates differ in their affinities for the enzymes.4. It is proposed that the brush border peptidases serve an analogous function in the terminal stages of protein digestion to that of the disaccharidases in the case of carbohydrate digestion and absorption.

Alkaline Phosphatase↗
PubMed1971

The isoenzymes of carbonic anhydrase: tissue, subcellular distribution and functional significance, with particular reference to the intestinal tract.

1. The total carbonic anhydrase activity in some guinea-pig tissues has been measured using a pH-stat procedure. Stomach, gall bladder, proximal colon and caecum all possess more carbonic anhydrase activity per unit amount of protein than does whole blood.2. The carbonic anhydrase activity of the small intestine is low. Reasons are given for supposing that activity found there is not entirely due to contamination by whole blood, and it is suggested that in this tissue the enzyme may be localized in some cell type other than the columnar absorbing cells.3. Evidence is presented which indicates that heavy metals interfere with the activity of the enzyme as measured in tissue homogenates.4. The distribution and concentration of the two major isoenzymes of carbonic anhydrase have been measured in different tissues. Blood and proximal colon contain both isoenzymes in comparable concentrations, the ratio of the concentration of the ;low activity' isoenzyme to that of the ;high activity' being about 2. The gastric mucosa contains much ;high activity' carbonic anhydrase, but only a negligible amount of the ;low activity' isoenzyme. In the caecal mucosa, the ;low activity' isoenzyme is predominant, the ratio of its concentration to that of the ;high activity' isoenzyme being about 9. It is also found that more than 1.5% of the protein in the caecal mucosa is accounted for as carbonic anhydrase enzymes.5. It is found that some 45% of the total carbonic anhydrase activity of sucrose homogenates of the guinea-pig colon is bound to particles. The activity is located mainly in the nuclear and microvillous fraction and in the ;high-speed supernatant' fraction. The form of enzyme bound is largely of the ;high activity' variety. When the tissue is homogenized in potassium chloride solutions less than 4% of the total activity is recovered in particulate fractions. The amount of activity which is bound to particulate fractions increases as the ionic strength or pH of the homogenate is lowered.6. The findings are discussed in relation to the possible physiological roles of the isoenzymes in tissues other than blood. Possible relationships between the presence of the enzymes and the metabolism and transport of ammonium and fatty acids are considered.

Animals↗
PubMed1971

Some properties of a preparation of rat colon perfused in vitro through the vascular bed.

1. A technique is described for the perfusion of the isolated colon of the rat, involving the infusion of an appropriate fluid through both the inferior and the superior mesenteric arteries. During neither the preparation nor the subsequent perfusion is the colon without an adequate supply of oxygen. The preparation remains histologically intact and metabolically viable and is capable of actively transporting ions for up to 5 hr.2. The addition of at least 3 g albumin/100 ml. perfusate is necessary to prevent the formation of large quantities of serosal exudate. With erythrocytes added to the vascular perfusate the preparation appears to be adequately oxygenated as judged by measurements of the rate of glycolysis. The mean rate of oxygen utilization over 4 hr is 9.2 +/- 0.3 (4) mumole. hr(-1).g(-1) fat free dry weight.3. Ion transport rates approaching those found in vivo are found only after the administration of an antihistamine substance to the colon donor rat before operation. In the absence of an antihistamine substance there appears to be an ultrafiltration of the plasma fluid into the lumen.4. Vasodilatory substances accumulate in the recycled perfusate. In a ;single pass' perfusion, the transport capacity of the preparation decreases at high perfusion pressures. It is suggested that this is due to some form of autoregulation whereby perfusate is shunted away from the epithelium into deeper layers as the pressure is increased.5. With CO(2) absent from the vascular infusate there is an increase in the net lumen to blood flux of total CO(2). This increased flux is accompanied by an equivalent amount of cation, comprising Na(+) and K(+) in the ratio of 12:1.6. The presence of ammonium in the lumen, a physiological constituent of the contents of rat distal colon in vivo, has a marked inhibitory effect upon the secretion of CO(2) into the contents of the lumen of the colon.

Animals↗