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D S Parsons

Publications and source records attributed to D S Parsons.

At least 127 records · Page 7Linked to original sources
PubMed1970

The purification and properties of carbonic anhydrases from guinea-pig erythrocytes and mucosae of the gastrointestinal tract.

Procedures for isolating carbonic anhydrase (EC 4.2.1.1) enzymes from the erythrocytes and the mucosae of the gastrointestinal tract of guinea pigs are described. From a haemolysate, haemoglobin was removed by the addition of ammonium sulphate, and also by two other methods, namely by gel filtration or by adsorption on DEAE-Sephadex. The crude enzyme thus obtained was resolved into the different isoenzymes by chromatography with DEAE-cellulose. From particle-free supernatants of homogenates of some gastrointestinal tissues, carbonic anhydrases were purified by ammonium sulphate fractionation, gel filtration, and ion-exchange chromatography with DEAE-cellulose. The major isoenzymes from blood, stomach, proximal colonic mucosa and caecal mucosa were homogeneous during ion-exchange chromatography, acrylamide-gel electrophoresis, and centrifugal examination. From these tissues, carbonic anhydrase was isolated as two major isoenzymes. They resemble the pairs of isoenzymes discovered in the bloods of other species. The carbon dioxide hydratase activity of one isoenzyme (;high activity' carbonic anhydrase) was 40 times that of the other isoenzyme (;low activity' carbonic anhydrase), as measured at a single substrate concentration. Two other minor components of the enzyme are also found in guinea-pig erythrocytes. All of the enzymes isolated had molecular weights of nearly 30000 (sedimentation equilibrium). ;High activity' carbonic anhydrases from blood and gastrointestinal tissues were indistinguishable according to some chemical, physical and kinetic measurements; similarly ;low activity' carbonic anhydrases from those tissues were indistinguishable. ;High activity' carbonic anhydrase was markedly different from the ;low activity' carbonic anhydrase with respect to its amino acid composition, chromatographic behaviour and isoelectric pH value. Marked differences were also found in the tissue concentrations of the major isoenzymes. It is suggested that the characteristic and selective distribution of the different forms of carbonic anhydrase in the guinea-pig tissues is related to the specific and different physiological functions of the enzymes.

Amino Acids↗
PubMed1968

A preparation of perfused small intestine for the study of absorption in amphibia.

1. A preparation of amphibian small intestine perfused through its vascular system is described. Vascular perfusion with a bicarbonate Ringer solution containing a colloid is used to control the composition of the environment of the submucosal faces of the absorbing cells and to carry away for collection any material extruded from these cells. Oxygenation of the mucosal cells is derived primarily from fluid circulated through the intestinal lumen. The preparation exhibits physiological properties of transport for periods of up to 5 hr. After 5 hr perfusion the epithelial cells show no signs of gross cellular damage when examined either by light or by electron microscopy.2. The relationship between the hydrostatic pressure at the mesenteric artery and the rate of perfusion through the vascular bed is substantially linear. The pressure-flow relationships in the mesenteric bed, including an apparent ;critical closing pressure', are primarily determined by the hydrostatic pressure in the intestinal lumen. Alterations in the hydrostatic pressure in the intestinal lumen also change the relative proportions of the vascular infusate which appear in the portal venous effluent and in the fluid exuded from the serosal surface of the preparation (;sweat'). Hydrostatic distension pressures above about 10 cm H(2)O reduce the rate of collection of fluid from the portal vein and increase the rate of collection of ;sweat'.3. An increase in the rate of vascular perfusion increases the total rate of glucose appearance although the glucose concentrations in both the portal effluent and the ;sweat' are reduced.4. The glucose translocation rate is related in an alinear saturable fashion to the luminal concentration of glucose. By making a correction for metabolic loss of glucose during its passage through the intestinal cell, the relationship existing between the lumen concentration and the uptake of the sugar by the mucosal cells has been calculated. This relationship is found to fit Michaelis-Menten type kinetics. The K(m) of the intestinal translocation process for glucose in Rana pipiens was 0.45 +/- 0.13 (4) muM. The mean V(max) was 137.5 +/- 35.3 (4) muM/hr/g fat-free dry wt.5. When phlorrhizin (10(-5)M) is added to the vascular perfusate, no inhibition of glucose transport is seen for at least 60 min. When strophanthin is added to the vascular perfusate (5 x 10(-5)M), a markedly greater inhibition of glucose transport is observed than when it is introduced to the luminal circulation.6. Earlier studies of the vascular perfusion of isolated small intestine are tabulated. The experimental findings are discussed in relation to a model of the mode of action of the epithelial cell for glucose transport.

Animals↗
PubMed1968

Disaccharide absorption by amphibian small intestine in vitro.

1. An account is given of the absorption of disaccharides by the small intestine of Rana temporaria, R. pipiens and Bufo vulgaris perfused in vitro through the vascular system. Maltase and trehalase activity are found in the intestine of all three species; very small amounts of sucrase are present in the intestine of R. pipiens but there is no evidence for the presence of lactase in any of the animals studied.2. During maltose absorption free glucose appears in the vascular effluent and in the intestinal lumen. Only very small quantities of disaccharide are found in the vascular effluent. The concentration of free glucose in the intestinal lumen during maltose absorption is not high enough to account for the rates of glucose transport observed. The rate of appearance of glucose in the vascular effluent is determined by the concentration of disaccharide in the luminal fluid, and hexose, free in solution in the lumen, is not an obligatory intermediate in the process of disaccharide absorption.3. For R. pipiens more than 90% of the maltase activity in the system is present in the intestinal wall and the rate of maltose hydrolysis by maltase, free in the intestinal lumen, is found to be inadequate to account for the rates of appearance of glucose observed to occur in the lumen and in the vascular effluent. It is not possible to wash away maltase activity from the intestinal wall.4. The kinetic properties of maltase and trehalase acting in situ are of the Michaelis-Menten type; the apparent K(m) is 2 mM for maltase, and 3 mM for trehalase.5. The relationship which exists between the rate of absorption of glucose and the concentration in the luminal fluid of either disaccharide or free glucose is of the Michaelis-Menten type. Expressed in molar units, the apparent K(m) for the glucose transport is about one fifth that of the disaccharidase. The maximum rate of glucose transport observed is less than the maximum rate of disaccharide hydrolysis. In R. pipiens equimolar concentrations in the intestinal lumen of the monomer free glucose, or of the dimer, maltose, yield approximately equal rates of transport of the free hexose.6. It is concluded that in the amphibian, either intestine disaccharide hydrolysis and glucose transport are functions of separate subcellular systems which spatially are very closely related, or that the hydrolysis and transport are different facets of the activity of a common system.

Animals↗
PubMed1966

The influence of thyroid gland on amino acid accumulation and protein synthesis by rat small intestine in vitro.

1. A method is described for measuring the accumulation of amino acids and also the extent to which the absorbed amino acids are incorporated into the tissue protein of rings of rat intestine in vitro.2. With this method rat jejunum was found to accumulate a mixture of amino acids to give a concentration in the tissue water which was 2-3 times that in the medium after 8 min incubation. More than 10% of the absorbed amino acids were found to be incorporated into protein.3. In surgically thyroidectomized animals there was no impairment of the capacity of the intestine to accumulate amino acids nor was there any loss of ability to incorporate these amino acids into protein. Similarly, treatment of the thyroidectomized animals with 3,5,3'-triiodothyronine (T(3)) did not produce any significant change either in the rate of accumulation or in the extent of the incorporation of the amino acids.4. The omission of glucose from the incubation medium was found to cause a large increase in the respiration of mucosal slices in the presence of the amino acid mixture. Thyroidectomy significantly reduced this high respiration rate. Treatment of thyroidectomized animals with T(3) restored the respiration to high value.

Amino Acids↗
PubMed1966

Amino acid accumulation and incorporation in rat intestine in vitro.

1. Rings of rat jejunum incubated in vitro accumulate a mixture of amino acids at a rate of about 3 mumoles/cm. hr. The rate of incorporation of the accumulated amino acid into the tissue protein corresponds to a rate of synthesis of 50% of the protein of the whole wall in five days.2. Replacement of the Na(+) in the NaCl of the incubation medium by choline or by Li(+) did not prevent amino acid accumulation by the tissue. However, replacement of the Na(+) by K(+) prevented the accumulation.3. The accumulation of amino acids by rat jejunum in vitro proceeded at normal rates not only in the presence in the incubation medium of oxygen tensions below 10 Torr but also in the presence of 2,4-dinitrophenol. Reasons are given for supposing that the findings are compatible with the view that the energy upon which depend the processes of amino acid accumulation by the tissue could be derived from the movement of ions across cellular boundaries.4. The amino acid incorporation into the tissue proteins was reduced to one tenth of the control rate in the presence of 2,4-dinitrophenol or by hypoxia so that the processes of incorporation depend upon energy derived from oxidative metabolism. In the presence of oligomycin the tissue respiration was depressed but the amino acid incorporation into the tissue protein was not inhibited. The view that the amino acid incorporation may be able to function with energy intermediates other than ATP is discussed.

Amino Acids↗