Glycosyl ureides in ruminant nutrition. 2. In vitro studies on the metabolism of glycosyl ureides and their free component molecules in rumen contents

In vitro validation of the lactose 13C-ureide breath test for equine orocaecal transit time measurement

REASONS FOR PERFORMING STUDY

Validation of a reliable, noninvasive clinical test for quantification of equine orocaecal transit time (OCTT) is required. This would facilitate an evidence-based approach to investigation and treatment of equine small intestinal disorders.

OBJECTIVES

1) Comparison of the lactose (13) C-ureide breath test (LUBT) with the hydrogen breath test (H(2) BT) for OCTT measurement. 2) Identification of the characteristics of gastrointestinal microbial glycosylureide hydrolase activity in vitro. 3) Production of an optimised protocol for the LUBT for in vivo measurement of equine OCTT.

HYPOTHESIS

Significant lactose (13) C-ureide ((13) C-LU) hydrolase activity is restricted to the large bowel. The rate of expiratory (13) CO(2) production after ingestion of the isotope will provide an indirect quantifiable measure of orocaecal transit rate. Requisite bacterial activity may be enhanced by a primer dose of unlabelled substrate as shown in Man.

METHODS

Combined LUBT and H(2) BT were performed in 8 healthy individuals. Analysis of sequential end expiratory breath samples was used to calculate OCTT and results compared. Digestion of (13) C-LU was investigated in vitro using fresh faecal material or intestinal aliquots collected post mortem. Isotopic fermentation rate was measured by rate of appearance of (13) CO(2) .

RESULTS

Peaks in expiratory (13) CO(2) occurred in all individuals after ingestion of the labelled test meal, whereas H(2) expiration was variable. Both faecal and intestinal microbial digestion of (13) C-LU were maximised by prior exposure to (12) C-LU. Induced bacterial glucoseureide hydrolase activity was significantly greater in the caecum than in the small intestine (n = 10, P<0.05).

CONCLUSIONS

Significant (13) C-LU digestion is restricted to the equine large intestine under normal conditions, and is enhanced by prior exposure to (12) C-LU, making (13) C-LU a suitable noninvasive marker of equine OCTT. The LUBT is more reliable than the H(2) BT for measurement of equine OCTT.

Application of the lactose 13C-ureide breath test for measurement of equine orocaecal transit time

REASONS FOR PERFORMING STUDY

Application of the lactose (13) C-ureide breath test (LUBT) for measurement of equine orocaecal transit time (OCTT) has not been reported previously. The ability to assess OCTT noninvasively, and to investigate its relationship to gastric emptying rate and small intestinal transit, would be of both clinical and research value.

OBJECTIVES

1) Assessment of the LUBT in healthy horses, with comparison of induced versus noninduced test protocols. 2) Application of a new dual stable isotope breath test (lactose (13) C-ureide and (13) C-octanoic acid) for gastrointestinal transit measurement.

HYPOTHESIS

The LUBT will allow quantification of equine OCTT, and test efficacy will be enhanced by prior administration of lactose (12) C-ureide as shown in vitro. The dual tracer breath test will permit simultaneous measurement of gastric emptying, OCTT and small bowel transit times.

METHODS

Induced and noninduced LUBTs were performed in 3 healthy mature horses in randomised order using a standard test meal and protocol. Combined LUBT and (13) C-octanoic acid breath tests ((13) C-OABT) were performed in 4 individuals on 4 occasions at weekly intervals. Expiratory isotopic recovery was modelled to allow generation of gastric emptying data, small bowel transit times and caecal transit parameters.

RESULTS

The induction protocol for the LUBT increased the rate and magnitude of expiratory (13) CO(2) significantly. Mean ± s.d. values for OCTT, caecal lag phase (t(lag) ) and caecal t(1/2) using the induced LUBT were 3.24 ± 0.65 h, 5.62 ± 1.22 h and 6.31 ± 1.21 h, respectively. Dual stable isotope tests resulted in the production of 2 discrete peaks in expiratory (13) CO(2) in 15/16 tests from which gastric t(1/2), OCTT and small bowel transit (SBT) parameters could be calculated.

CONCLUSIONS

The induced LUBT provides a reliable noninvasive measure of equine OCTT and can be paired with the (13) C-OABT to provide further information about small intestinal motility.

Evaluation of the necessity of induction for lactose-[15N, 15N]-ureide to study the colonic ammonia metabolism

OBJECTIVE

Stable isotope labelled glycosyl ureides have been described as non-invasive markers for measurement of gastrointestinal processes. Lactose-[13C]-ureide is used for the evaluation of orocaecal transit time (OCTT), whereas lactose-[15N, 15N]-ureide is used to study the fate of the NH3 metabolism in the colon. Induction with unlabelled lactose ureide is necessary for the determination of the OCTT. In the present study, the effect of a preceding induction on the fate of 15NH3 in the colon was evaluated.

MATERIAL AND METHODS

Ten healthy volunteers performed two tests: the first test without induction, the second test one week later with induction, i.e. administration of 1 g lactose ureide the evening before the test. Each test consisted of a fractionated 24-h urine collection after the subjects had received a pancake test meal labelled with 75 mg lactose-[15N, 15N]-ureide. All samples were analysed for [15N]-content by combustion IRMS (isotope ratio mass spectrometry) and results were expressed as a percentage of the administered dose.

RESULTS

No significant differences were found in the percentage dose 15N excreted in the different urine fractions between the test without and with preceding induction. The cumulative excretion rates of the 15N-isotope after 24 h were 44.35% without induction and 44.27% with induction.

CONCLUSIONS

The results in this study show that predosing with unlabelled lactose ureide is unnecessary for the evaluation of the ammonia metabolism in the colon by means of lactose-[15N, 15N]-ureide.

Relationship between serum N-carbamoyl-beta-D-glucopyranosylamine level and renal failure

A statistical investigation was carried out on the distribution of serum N-carbamoyl-beta-D-glucopyranosylamine (NCG) among various patient groups. The serum NCG levels of patients treated in the departments of hemodialysis (131 +/- microM), nephritic syndrome (47 +/- 54 microM), and diabetes mellitus (55 +/- 70 microM) were significantly higher (p < 0.01) than those in other internal disease patients (18 +/- 22 microM) and healthy volunteers (6 +/- 22 microM). The serum NCG level was greatly reduced by hemodialysis therapy, however a return to initial NCG levels was observed within about one week. These results indicate that a high serum NCG level is a feature of renal failure patients, and a relationship was demonstrated between hyperuremia and NCG formation and accumulation in blood.

Metabolism of glycosyl ureides by human intestinal brush border enzymes

13C-labeled glycosyl ureides were recently proposed as a new marker of the orocecal transit time: after passing the small bowel the sugar-urea bond is split by bacterial allantoicase. Further degradation results in 13CO2 which can be measured in the exhaled breath. The aim of this study was to detect an eventual allantoicase-like activity in the human gut and to elucidate the metabolism of glycosyl ureides by human intestinal brush border enzymes. Biopsies of 15 duodenal specimen and 6 colon specimen were homogenised and incubated with several disaccharides and their corresponding disaccharide ureides under various experimental conditions. Hydrolysis of the sugar-urea bond could not be observed neither in the small bowel nor in the colon. However, the conjugation between the two sugars was split. In a modified Dahlqvist assay lactase showed the same kinetics with lactose and lactose ureide as substrates whereas maltose showed a significantly 2.6-fold higher affinity to maltase than maltose ureide (P < 0.001). No major difference between these two substrates could be detected when total maltase activity was inhibited by acarbose. In summary, the human gut tissue possesses no allantoicase-like activity. Therefore, glycosyl ureides seem to be appropriate substances to test the orocecal transit time.

Studies of rumen function in an in vitro continuous culture system

An in vitro continuous culture system to simulate processes in the rumen is described. This comprises a culture vessel (Fig. 2) fed continuously with pelleted solid feed (Fig. 1); artificial saliva also enters the vessel while effluent leaves it continuously in two streams, one is filtered and the other simply overflows (Fig. 1). In this way liquid and solid turnover times may be manipulated independently; in a trial experiment the former and latter were 6.3% and 3.4% per hour respectively. In four replicated experiments a steady-state was achieved in 5-6 days with a feed of barley, tapioca, fishmeal, urea and straw and maintained for a further 7 to 14 days. During a steady-state period, rumen characteristics in terms of protozoal count, pH, redox potential, total volatile fatty acid concentration and ammonia concentration were stable and similar to those found in the rumens of animals given similar diets. Flows of microbial N compounds were assessed by infused Na2H32PO4 with the artificial saliva and estimating the 32P label incorporated in bacterial fractions. The system demonstrably provided a satisfactory means for studying many aspects of rumen function.

A comparison of the chemical composition of mixed bacteria harvested from the liquid and solid fractions of rumen digesta

Steers, equipped with simple rumen cannulas, were given diets of approximately equal parts of rolled barley and straw supplemented with urea. The diets provided sufficient estimated rumen degradable nitrogen (RDN; RDN:metabolizable energy values of 1:3) to maintain maximum microbial synthesis. In some experiments Na235SO4 was introduced into the rumen to label microbial protein. Rumen digesta samples were taken before feeding and mixed rumen bacteria were separated from the solid (solid-associated bacteria; SAB) and liquid (liquid-associated bacteria; LAB) fractions of digesta. The most effective method of removing SAB from the fibre was a combination of homogenizing and pummelling. This process did not affect the physical form or chemical composition of the bacteria. Samples of SAB contained significantly (P less than or equal to at least 0.05) less ash, total N, RNA and diaminopimelic acid (DAP) and significantly (P less than or equal to 0.01) more lipid than samples of LAB. Concentrations (g/kg dry matter) of ash, total N, RNA, DAP and lipid in SAB were approximately 87, 70, 35, 2.2 and 245 respectively. Corresponding values for LAB were 157, 80, 50, 3.8 and 124 respectively. RNA-N:total N and DAP-N:total N values in SAB were significantly lower than those in LAB (P less than or equal to 0.05 and 0.02 respectively). 35S:total N values were similar in both groups of bacteria. The importance of differences in constituent:total N values in the two groups of bacteria in relation to their use as indices of microbial protein synthesis is discussed.

Glycosyl ureides in ruminant nutrition. 3. In vivo studies on the metabolism of glycosyl ureides and corresponding mixtures of their free component molecules

1. Steers and sheep were given basal diets of barley and straw (1:1, w/w), usually containing urea, which for certain experimental periods were supplemented with pure glucosyl urea (GU), pure lactosyl urea (LU) or a product prepared from whey concentrate (EW) which contained 65-80% of the lactose in the form of LU. 2. On the morning of an experiment ureide (or EW) was omitted from the feed and a dose of either ureide (or EW) or equivalent amounts of free lactose and urea (L + U) was added to the rumen within 30 min of feeding, together in some experiments with polyethylene glycol (PEG) as a fluid-phase marker. Samples of rumen contents, and in some experiments abomasal contents, were taken at intervals for up to 8 h. 3. For both steers and sheep given GU and LU for the first time (unadapted animals) there was little or no accumulation of ammonia in the rumen or cleavage of the sugar-urea bond. Galactose was, however, slowly liberated from LU. 4. For steers and sheep which had been given GU, LU or EW for approximately 7-10 d or more (adapted animals) some accumulation of ammonia occurred after adding GU or LU to the rumen, but for LU it occurred less rapidly and to a lower peak concentration than when L + U was added. In adapted animals cleavage of the sugar-urea bond in LU was virtually complete in 2-4 h. Degradation of the components of L + U was virtually complete within 1 h. 5. Recovery at the abomasum of ureide (present either as GU or LU) estimated from ureide: PEG values, appeared to be complete in experiments with unadapted sheep given a dose of EW. In adapted sheep only very small amounts of ureide in an EW dose (on average 6%) entered the abomasum undegraded. Amounts lost in this way appeared to be positively correlated with the rate of fluid turnover in the rumen.