Impairment of glycine absorption by glucose and galactose in man

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

Dog bites man or man bites dog? The enigma of the amino acid conjugations

The proposition posed is that the value of amino acid conjugation to the organism is not, as in the traditional view, to use amino acids for the detoxication of aromatic acids. Rather, the converse is more likely, to use aromatic acids that originate from the diet and gut microbiota to assist in the regulation of body stores of amino acids, such as glycine, glutamate, and, in certain invertebrates, arginine, that are key neurotransmitters in the central nervous system (CNS). As such, the amino acid conjugations are not so much detoxication reactions, rather they are homeostatic and neuroregulatory processes. Experimental data have been culled in support of this hypothesis from a broad range of scientific and clinical literature. Such data include the low detoxication value of amino acid conjugations and the Janus nature of certain amino acids that are both neurotransmitters and apparent conjugating agents. Amino acid scavenging mechanisms in blood deplete brain amino acids. Amino acids glutamate and glycine when trafficked from brain are metabolized to conjugates of aromatic acids in hepatic mitochondria and then irreversibly excreted into urine. This process is used clinically to deplete excess nitrogen in cases of urea cycle enzymopathies through excretion of glycine or glutamine as their aromatic acid conjugates. Untoward effects of high-dose phenylacetic acid surround CNS toxicity. There appears to be a relationship between extent of glycine scavenging by benzoic acid and psychomotor function. Glycine and glutamine scavenging by conjugation with aromatic acids may have important psychosomatic consequences that link diet to health, wellbeing, and disease.

Splanchnic fluxes of amino acids after duodenal infusion of carbohydrate solutions containing free amino acids or oligopeptides in the non-anaesthetized pig

Seven non-anaesthetized pigs (mean body-weight 64.6 kg) were used to study the intestinal absorption and hepatic metabolism of glucose and amino acids (AA) using carbohydrate solutions (maltose dextrin; 440 g/2 I), containing 110 g of either an enzymic milk-protein hydrolysate (PEP) with a large percentage of small peptides (about 50% with less than five AA residues) and very few free AA (8%) or a mixture of free AA (AAL) with an identical pattern, infused intraduodenally. Each pig was previously fitted under anaesthesia with electromagnetic flow probes around the portal vein and the hepatic artery, and with permanent catheters in the portal vein, carotid artery, one hepatic vein and the duodenum. Each solution was infused for 1 h after a fasting period (18 h) and each pig received both solutions at 8 d intervals. The observation period lasted 8 h. For most AA (his, lys, phe, thr, arg, tyr, pro) the absorption rate after infusion of PEP was significantly higher than after that of AAL during the 1st hour, but the differences quickly disappeared. After 8 h, the only differences concerned his and tyr (PEP > AAL) and met, glu and asp (AAL > PEP). There was a large uptake of blood AA by gut-wall cells, higher after AAL infusion than after PEP infusion, particularly for branched-chain AA (BCAA). The absorption of ammonia-nitrogen after both infusions was equivalent to two-thirds of urea-N passing from blood to intestinal tissues and lumen. Glucose absorbed within 8 h represented only 76% (PEP) or 69% (AAL) of the infused amounts. The cumulative hepatic total AA (TAA) uptake increased from 13 to 27% of the infused amounts between the 1st and the 8th hour after PEP infusion, and from 8 to 31% after AAL infusion. Most essential AA were largely taken up by the liver, with the exception of met (PEP) and thr and of BCAA, which were poorly retained for both solutions; there was a high uptake of ala and gly, and a release of asp, glu, and gln. Urea-N released by the liver within 8 h was equivalent to 23-25% absorbed amino-N and to around 1.5 times ammonia-N taken up by the liver within 8 h. Glucose was highly taken up by the liver during the first hours then released, the total uptake within 8 h representing about half the absorbed amount.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutual inhibition of sugars and amino acid intestinal absorption

1. Intestinal absorption of sugars shows interaction with amino acids and vice versa. 2. The percentage of inhibition induced by a substrate depends on the type of substrate and the substance affected. 3. The mutual inhibition of monosaccharides and amino acids can be explained by five different hypotheses. 4. The most accepted hypotheses are the "allosteric interaction" and the "accelerated efflux". 5. From the results in the literature it is difficult to decide which of the two hypotheses is the more accurate.

The response to potassium of the Na-K pump ATPase in low-K red blood cells from cattle at birth and in later life

It is shown that in low-K red blood cells of cattle the apparent affinity for K(1/Kapp K) at an inhibitory site of the Na-K ATPase increases markedly during the first 3 months of life. This site probably is the Na accepting site at the internal membrane surface and the change in Kapp K reflects an increase in KNa/KK, the ratio of the true dissociation constants. This effect may explain the concomitant fall in cellular K concentration.

Jejunal absorption rates of glucose and glycine in post-infective tropical malabsorption

In a prospective study jejunal absorption rates of glucose (from a 200 mmol 1-1 solution) and glycine (from a 100 mmol 1-1 solution) were measured, using a perfusion technique, in nine patients with clinical evidence of post-infective tropical malabsorption (TM group) and in 10 others with trivial symptoms who had also visited the tropics (control group); all were of northern European origin. Seven and one in the two groups, respectively, had Giardia lamblia infections. Mean glucose absorption rate was significantly impaired by TM (P less than 0.01); mean rate for glycine was also depressed but not significantly. Presence of G. lamblia did not affect the severity of malabsorption although numbers studied were small.

A change in the internal affinity of LK goat red-cell sodium pumps induced by high pH

The K inhibition of ouabain-sensitive ATPase activity of LK goat red cell membranes is greatly reduced at high pH. This effect is reversible, and specific, since the apparent affinities for ATP, ouabain or external K do not alter. Anti-L-treated membranes show a similar alkali-induced affinity change, but have a lower pH optimum.

Rates and mechanisms of glucose, galactose, and xylose absorption in man in vivo

Jejunal absorption-rates of glucose, galactose, and xylose were determined consecutively, using a double-lumen tube perfusion technique, in 6 Arab subjects in vivo; perfusion solutions contained 200 mmol/l of each monosaccharide. Mean xylose absorption rate was significantly lower than that for glucose (P less than 0.001) and galactose (P less than 0.001). In order to investigate evidence that glucose and xylose share the same jejunal transfer mechanism, their absorption-rates when given alone and together, at high concentrations, were determined in 6 other Arabs; perfusion solutions contained (1) 200 mmol/l glucose, (2) 100 mmol/l xylose, and (3) 200 mmol/l glucose + 100 mmol/l xylose. Rates of glucose and xylose absorption were not significantly influenced by the other monosaccharide. This study does not therefore confirm existence of a common transfer mechanism. A significant correlation between absorption rate of glucose and weight of xylose excreted after a 25 g oral load (P less than 0.05) was demonstrated.

Absorption of xylose, glucose, glycine, and folic (pteroylglutamic) acid in Zambian Africans with anaemia

Rates of glucose, glycine, and folic (pteroylglutamic) acid absorption were determined for a 30 cm jejunal segment in vivo, with a double-lumen tube perfusion system, in 10 Zambian African women with a mean haemoglobin concentration of 5-1 (3-5-9-2) g/dl. In four the anaemia was megaloblastic (due to folate deficiency) and in six hypochromic. Perfusion solutions contained (1) glucose 200 mmol/1, (2) glycine 100 mmol/1, and (3) folic acid 250 mug/1. D-xylose absorption after a 25 g oral load was determined in them, and also in 18 additional patients (11 had megaloblastic and seven either hypochromic or haemolytic anaemia). Xylose absorption tests were significantly impaired in the patients with megaloblastic compared with hypochromic or haemolytic anaemia (P less than 0-001); those with untreated megaloblastic anaemia had a greater abnormality than those who had started treatment. Mean glucose, glycine, and folic acid absorption rates were similar to those in controls, and the rates in patients with megaloblastic and hypochromic anaemia were not significantly different. Correlation between glucose absorption rate and xylose excretion was, however, significantly (P less than 0-02). If more patients had been studied it seems likely therefore that a significant impairment of glucose absorption rate in the presence of megaloblastic anaemia would also have been demonstrated. In this investigation anaemia per se did not affect glucose, glycine, or folic acid absorption rates or xylose absorption, but xylose absorption was reduced in patients with megaloblastic anaemia. That abnormality was probably related to folate deficiency, and the underlying mechanism seems to be different from that causing impairment of monosaccharide absorption in patients with systemic bacterial infections. Mean glycine and folic acid absorption rates were not altered by megaloblastic anaemia, indicating that folate deficiency does not cause a general depression of absorption.

Impairment of jejunal absorption rate of carnosine by glycylglycine in man in vivo

Using a double-lumen tube jejunal perfusion system in vivo, the mutual effects of carnosine (beta-alanyl-L-histidine) and glycylglycine on their respective absorption rates have been studied in six Zambian African adults. Data on the effect of the constituent amino-acids of carnosine on glycylglycine absorption rate have similarly been obtained. The solutions infused in each subject contained (A) carnosine (50 mmol l.-1), (B) carnosine (50 mmol l.-1) and glycylglycine (50 mmol l.-1), (C) glycylglycine (50 mmol l.-1), and (D) glycylglycine (50 mmol l.-1), L-histidine (50 mmol l.-1) and beta-alanine (50 mmol l.-1). Glycylglycine produced a significant impairment in the mean rate of histidine absorption from carnosine (P less than 0-01). However, carnosine did not have a significant effect on the mean rate of glycine absorption from glycylglycine. Mean rate of histidine absorption from solution D was significantly higher than that from solution A (P less than 0-01). Mean rate of glycine absorption from glycylglycine was not significantly different during infusion of solutions B, C, and D. The results are consistent with the concept that carnosine on glycylglycine is probably because the affinity of mechanism; the lack of influence of carnosine on glycylglycine is probably because the affinity of carnosine for the dipeptide uptake mechanism is relatively low. A gross difference has been shown between mean absorption rate of histidine from free L-histidine (solution D) (25-8 mmol h-1) and when it is given in the form of carnosine in the presence of another dipeptide (solution B) (8-7 mmol h-1); that emphasizes the complexity of amino acid and peptide interaction during absorption, which must be important in nutrition.

Interaction between sugars and amino acids in intestinal absorption by rat, in vivo

The inhibitory action of L-leucine on the intestinal absorption of D-glucose and D-galactose, as well as the inhibitory action of D-galactose on the absorption of L-leucine at various concentrations by rat small intestine has been studied. The further effect was more clearly evidenced when the medium was perfunded through the intestine in a closed circuit system using a peristaltic pump.

Absorption of L-histidine and glucose from the jejunum segment of the pig and its diurnal fluctuation

1. Flow rate of digesta and its component in the upper jejunum, and the absorption of L-histidine and glucose from the jejunum segment were measured in pigs fitted with three simple cannulas. The pigs were fed once faily at 08.30 hours. 2. Maximum flow of digesta was obtained in the period 10.00-10.30 hours; the flow rate decreased with time after feeding, reaching a minimum in the period 22.00-22.30 hours. 3. The absorption rate for L-histidine and glucose increased in a hyperbolic manner with increasing concentrations of infused test material, which ranged from 2.5 to 20 g/1 for each material. 4. L-histidine and glucose were absorbed nearly independently when perfused in combination. The absorption rates for glucose were significantly (P less than 0.01) greater than the corresponding rates for L-histidine at each concentration of infusate. 5. The absorption of both L-histidine and glucose expressed as a percentage of the amounts in the perfusate decreased with increasing flow rate of perfusate, from 400 to 800 ml/h. The increase in flow rate from 400 to 800 ml/h was associated with a 20% increase in L-histidine absorption rate; there was a 30% increased in glucose absorption rate when the flow rate was increased to 600 ml/h, but no further incrase at 800 ml/h. 6. The absorption of both L-histidine and glucose decreased with time after feeding; the absorption rates for L-histidine and glucose measured for the period 10.00-22-30 hours were 126 and 133%, respectively, of those measured for the period 22.00-22.30 hours.

Jejunal absorption rates of glucose, glycine and glycylglycine in Zambian African adults with malnutrition

1. Absorption rates of glucose (from a 200 mM solution), glycine (from a 100 mM solution), and glycylglycine (from a 50 mM solution) have been estimated in six Zambian African adults with clinical evidence of malnutrition. A double-lumen tube technique was used to determine absorption rates from a 300 mm section of jejunum in vivo.

2. Two of the subjects had ileal tuberculosis and Kaposi's sarcoma respectively. A third probably had abdominal tuberculosis. Three of them had pellagra. Mean serum albumin concentration was 24 (14–43) g/l. Absorption rates have been compared with those in Zambian Africans (control subjects), previously studied, who had no clinical evidence of malnutrition, systemic infection or of gastrointestinal disease.

3. Mean glucose, glycine and glycylglycine absorption rates in the malnourished subjects were not significantly different from those in the control subjects. Mean net water absorption rate from the glucose solution was similar in the malnourished subjects and controls; during the glycine and glycylglycine perfusions the mean net absorption rate was, however, significantly lower in the malnourished subjects (P < 0.01 and P < 0.05 respectively); mean net water transfer during the glycine perfusions was towards the jejunal lumen in the malnourished subjects. One subject with pellagra had an abnormal excretion of D-xylose after a 25 g oral load; all other tests were normal.

4. It seems probable that malnutrition must be very severe, with jejunal mucosal abnormalities, before absorption rates of glucose, glycine and glycylglycine are significantly altered. The present study does not support the view that subclinical malnutrition is important in producing malabsorption of dietary components in Zambian African subjects. Systemic bacterial infections, and raised serum γ-globulin and immunoglobulin IgG concentrations have previously been associated with an impairment of glucose absorption rate in Zambian African subjects; those factors seem much more likely than subclinical malnutrition to be relevant, in the context of absorption, in the pathogenesis of overt malnutrition.

Reaction of human small intestine to an intraluminal tube and its importance in jejunal perfusion studies

A double-lumen tube with a mercury weight attached to its distal end was positioned in the small intestine of six subjects who were to undergo an elective laparotomy. The tube and the technique for its localization were exactly similar to those used in previous intestinal perfusion experiments. Marked ;concertinaing', or gathering of the small intestine proximal to the mercury weight, was seen at laparotomy in all six subjects. The ratio between the mean intestinal length from the mercury weight to the ligament of Treitz after removal of the tube to that with the tube in situ was 3.0 (2.5-3.8). Mean total jejuno-ileal length was 421 (320-521) cm. In five of the six subjects the distal end of the tube was more than half way between the ligament of Treitz and the ileo-caecal valve although it was only 100-120 (mean 108) cm from the incisor teeth. Segmental perfusion studies of the human jejunum involve a much longer length of small intestine than is generally assumed-by a factor of approximately 3. Such studies assess absorption rates over a substantial proportion of the small intestine and are not confined to a short segment of proximal jejunum. The nutritional significance of such studies in which amino acids, peptides, and carbohydrates are investigated is increased. Studies designed to measure absorption rates from the ileum should be treated with caution as part or all of the perfusion segment may be past the ileo-caecal valve.

Effect of glycylglycine and glycine on jejunal absorption rate of L-histidine in man in vivo

1. Using a double-lumen tube jejunal perfusion system in vivo, the effects of glycylglycine and glycine on the jejunal transfer rate of L-histidine have been studied in a 30 cm jejunal segment, in six Zambian African men. Data on the effect of L-histidine on glycylglycine transfer rate have also been obtained. The solutions perfused in each subject contained (A) L-histidine (100 m-mole l.(-1)), (B) L-histidine (100 m-mole l.(-1)) and glycylglycine (50 m-mole l.(-1)), (C) glycylglycine (50 m-mole l.(-1)), and (D) L-histidine (100 m-mole l.(-1)) and glycine (100 m-mole l.(-1)).2. Whereas the presence of glycine in the perfusing fluid significantly impaired the absorption rate of L-histidine (P < 0.01), glycylglycine had no significant effect. The presence of L-histidine in the perfusing fluid had no significant effect on the rate of glycylglycine absorption. When compared with results for glycine absorption rate in twelve Zambian African subjects in another study, the presence of L-histidine produced a significant impairment in the rate of glycine absorption (P < 0.001).3. The results demonstrate a mutual inhibition between the rates of L-histidine and glycine absorption. The lack of a similar inhibition between L-histidine and glycylglycine is consistent with the concept of independent transfer mechanisms for amino acids and dipeptides in the human jejunum.

Lactose absorption kinetics in Zambian African subjects

1. Using a double-lumen tube perfusion system, solutions of lactose (50, 125 and 250 mmol/l) were introduced into the upper jejunum of six Zambian African subjects. By reference to a non-absorbable marker, polyethylene glycol, mol. wt 4000, the rates of absorption of lactose from each solution were calculated for a 300 mm jejunal segment.

2. In three subjects total lactase activity of the jejunal mucosa and brush-border lactase and other disaccharidase activities were estimated. The jejunal total and brush-border lactase activities were low. Jejunal morphology was normal for African subjects.

3. All subjects suffered abdominal colic and diarrhoea during and after the lactose perfusions. The kinetic curves for lactose were very shallow, and with all perfused solutions, there was a net movement of water into the jejunal lumen. The limited number of subjects, and the low and narrow range of enzyme activity, did not permit correlation between lactose absorption rate and lactase activity.

4. In Zambian African subjects with adult hypolactasia, the jejunal mucosa absorbs a very small proportion of the perfused lactose.

Independent jejunal mechanisms for glycine and glycylglycine transfer in man in vivo

1. Rates of absorption of glycine and glycylglycine from a 300 mm jejunal segment were compared in vivo when those compounds were given alone or together to six Zambian African subjects who had no clinical evidence of malnutrition or of gastro-intestinal disease. Solutions containing (A) glycine (100 mmol/1), (B) glycine (100 mmol/l)+glycylglycine (50 mmol/l), and (C) glycylglycine (50 mmol/l) were infused into the upper jejunum by means of a double-lumen tube perfusion system.

2. Rate of absorption of glycine was significantly higher from the glycylglycine solution (C) than from the glycine solution (A). Glycine absorption rate from solution B (glycine+glycylglycine) was very similar to the sum of absorption rates of glycine from solutions A and C in each subject. Luminal disappearance rate of glycylglycine from solutions C and B were very similar; however, the rate was significantly greater than the total glycine absorption rate from solution C and indicates back-diffusion of glycine into the lumen after glycylglycine hydrolysis.

3. The results are interpreted as indicating that the transport mechanisms for glycine and glycylglycine in man are partly, and possibly wholly, separate.

Increased glycine absorption rate associated with acute bacterial infections in man

1. Glycine absorption rate from a 300 mm jejunal segment was determined in vivo in four Zambian African subjects with acute, and four with chronic, respiratory infections. Glycine solutions (100, 150 and 250 mmol/l) were perfused, by means of a double-lumen tube technique. The results were compared with those for four relatively normal Zambian African subjects (‘reference’ group) previously studied. The group with acute-infections had a significantly higher mean absorption rate than the reference or chronic-infection group.

2. Glycine absorption results from a 100 mmol/l glycine solution in an additional twenty-four Zambian African subjects have also been analysed. When results for the thirty-six subjects were combined, those with acute bacterial infections had a significantly higher mean absorption rate than the normal subjects or those with chronic infections. For the twenty-one normal subjects there was a significant positive correlation between the individual absorption rates and serum total globulin and γ-globulin concentrations.

3. It seems likely that the rapid catabolism of protein associated with infection is counteracted by an increase in amino acid absorption rate. In subjects on a low-protein diet that mechanism would be limited. The deterioration in nutritional status during infections in developing countries could therefore be partly explained by the present observation.

Glucose absorption kinetics in Zambian African patients with and without systemic bacterial infections

Using a double-lumen tube perfusion system, solutions of glucose (1.0, 2.5, and 5.0 g 100ml(-1)) have been perfused into the upper jejunum of 22 Zambian African subjects in order to study their glucose absorption kinetics. None of them had clinical evidence of malnutrition or intestinal disease. In 10 there was no evidence of an infective disease (;normal' group); seven had tuberculosis; five had acute bacterial infections. The mean serum albumin concentration was significantly lower in those with infections; the mean total and gamma-globulin concentrations were significantly higher in the tuberculosis group. There was good reproducibility in triplicate assessments of glucose and water absorption rates in the individual subjects. Despite a wide scatter, the mean glucose kinetic curves were significantly flatter in those with infections than in the normal group (p<0.02). There was a significant association between glucose and water absorption rates in the individuals. D-xylose absorption was estimated in 11 subjects and there was a significant correlation between that and the glucose absorption rate. Jejunal morphology (n=9) and disaccharidase concentrations (n=6) were normal for African subjects and there were no significant associations between either of those and the absorption rates. Galactose absorption kinetics have been studied in an additional four relatively normal Zambian Africans. This study suggests that systemic bacterial infections can produce malabsorption. This may be relevant to the weight loss in patients with pulmonary tuberculosis and also to the aetiology of kwashiorkor.