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PubMed1976

Pyrophosphatase and glucuronosyltransferase in microsomal UDPglucuronic-acid metabolism in the rat liver.

1. A radiochemical method for the studies on the microsomal UDPglucuronic acid metabolism has been developed. 2. The rat liver microsomes caused a rapid hydrolysis of UDPglucuronic acid to D-glucuronic acid 1-phosphate and further although much slower to free D-glucuronic acid. In Tris-HCl buffer (pH 7.4) they were produced in ratio 72 : 1. No other metabolites were found in measurable amounts. The pyrophosphatase splitting UDPglucuronic acid showed a pH optimum at 8.9, but the liberation of D-glucuronic acid from UDPglucuronic acid had two pH maxima (pH 3.5 and 8.5). EDTA appeared to be less powerful inhibitor of pyrophosphatase than previously suggested. About 25 per cent of the UDPglucuronic acid hydrolyzing activity was still remaining in the presence of 10 mM EDTA. D-Glucaro-1,4-lactone was found to have a slight inhibitory action on the pyrophosphatase activity. Citrate inhibited powerfully the hydrolysis of UDPglucuronic acid and the liberation of free D-glucuronic acid. Phosphate was also inhibitory. 3. In the presence of an exogenous UDPglucuronosyltransferase substrate, 4-nitrophenol, the formation of D-glucuronic acid 1-phosphate and free D-glucuronic acid were slightly reduced, and D-glucuronic acid 1-phosphate, 4-nitrophenylglucuronide and free D-glucuronic acid were produced in ratio 78 : 23 : 1. When 10 mM EDTA was added to diminish the hydrolytic consumption of the glucuronyl donor substrate, the corresponding ratio was still as unfavorable as 19 : 2.6 : 1. The measurable activity of UDPglucuronosyltransferase was lower in the presence of phosphate or citrate than in Tris-HCl buffer, although they protected the glucuronyl donor substrate against hydrolysis. 4. The results indicate that even in the presence of added glucuronyl acceptor substrate the hydrolysis of UDPglucuronic acid predominates the conjugation in rat liver microsomes. The rate of the hydrolysis of UDPglucuronic acid is quite considerable even in the presence of EDTA, and it is recommended to control the UDPglucuronic acid pyrophosphatase activity when UDPglucuronosyltransferase and glucuronidation reactions are studied. Free D-glucuronic acid appears to be produced from UDPglucuronic acid for further use via D-glucuronic acid 1-phosphate, the rate-limiting step being the hydrolysis of this intermediate. UDP-glucuronosyltransferase, glucuronides of either endogenous or exogenous aglycones and beta-glucuronidase have only a minor role in this respect in rat liver microsomes.

Animals↗
PubMed1981

A kinetic mechanism for modulation of the activity of microsomal UDP-glucuronyltransferase by phospholipids. Effects of lysophosphatidylcholines.

The affinity of delipidated microsomal UDP-glucuronyltransferase (EC 2.3.1.17) for UDP is greater than that for UDP-glucuronic acid. Measurement of KIglucuronic acid reveals that glucuronic acid binds to the enzyme. Hence, the difference in affinity of the enzyme for UDP versus UDP-glucuronic acid indicates that inherent binding energy for interactions between enzyme and this substrate is used for purposes other than enhancing binding. A reasonable interpretation of these data is that the binding of UDP-glucuronic acid to enzyme requires distortion of the substrate and/or the enzyme. Inherent binding energy due to interactions between enzyme and UDP and glucuronic acid is utilized to effect such distortions. This type of mechanism can cause significant rate enhancement. Phospholipid activators of UDP-glucuronyltransferase activate by amplifying this basic mechanism. Thus, addition of various species of lysophosphatidylcholine to the delipidated enzyme increase the activity at Vmax and enhance the affinity for UDP, glucuronic acid, and UDP-glucuronic acid. However, activators enhance the affinity of the enzyme for UDP-glucuronic acid to a significantly smaller extent than they enhance affinity for the UDP and glucuronic acid portions of the substrate. Calculations of the amount of binding energy for interactions between enzyme and UDP-glucuronic acid that can be used for stimulating activities at Vmax yield values in agreement with the observed enhancement of activities at Vmax for enzyme reconstituted with various types of lysophosphatidylcholine.

Animals↗
PubMed1969

Aldohexuronic acid catabolism by a soil Aeromonas.

Bacteria which utilize mannuronic acid as an energy source were isolated from nature. One of the organisms, identified as a member of the genus Aeromonas, used glucuronate, galacturonate, and mannuronate as the sole source of carbon and energy. Glucuronate- and galacturonate-grown resting cells oxidized both glucuronate and galacturonate rapidly, but mannuronate slowly. Mannuronate-grown cells oxidized all three rapidly, with the rate of mannuronate utilization somewhat lower. Cell-free extracts from glucuronate-, galacturonate-, and mannuronate-grown Aeromonas C11-2B contained glucuronate and galacturonate isomerases, fructuronate, tagaturonate, and mannuronate reductases, and mannonate and altronate dehydratases, with the exception of glucuronate-grown cells which lacked altronate dehydratase. Thus, the pathway for glucuronate and galacturonate catabolism for Aeromonas was identical to Escherichia coli. Glucuronate and galacturonate were isomerized to d-fructuronate and d-tagaturonate which were then reduced by reduced nicotinamide adenine dinucleotide to d-mannonate and d-altronate, respectively. The hexonic acids were dehydrated to 2-keto-3-deoxy gluconate which was phosphorylated by adenosine triphosphate to 2-keto-3-deoxy-6-phospho gluconate. The latter was then cleaved to pyruvate and glyceraldehyde-3-phosphate. Mannuronate was reduced directly to d-mannonate by a reduced nicotinamide adenine dinucleotide phosphate-linked oxidoreductase. d-Mannonate was then further broken down as in the glucuronate pathway. The mannuronate reducing enzyme, for which the name d-mannonate:nicotinamide adenine dinucleotide (phosphate) oxidoreductase (d-mannuronate-forming) was proposed, was shown to be distinct from altronate and mannoate oxidoreductases. This is the first report of a bacterial oxidoreductase which reduces an aldohexuronic acid to a hexonic acid. The enzyme should prove to be a useful analytical tool for determining mannuronate in the presence of other uronic acids.

Aeromonas↗
PubMed1988

Formation of UDP-Xylose and Xyloglucan in Soybean Golgi Membranes.

Soybean (Glycine max) membranes co-equilibrating with Golgi vesicles in linear sucrose gradients contained UDP-glucuronate carboxy-lyase and xyloglucan synthase activities. Digitonin solubilized and increased the activity of the membrane-bound UDP-glucuronate carboxy-lyase. UDP-xylose did not inhibit the transport of UDP-glucuronate into the lumen of Golgi vesicles but repressed the decarboxylation of the translocated UDP-glucuronate. The results suggest that UDP-glucuronate is transported into the vesicles by a specific carrier and decarboxylated to UDP-xylose within the lumen. On incubation of UDP-[(14)C]glucuronate with Golgi membranes in the presence of UDP-glucose, [(14)C]xylose-labeled xyloglucan was formed. Although the K(m) value of UDP-glucuronate for the decarboxylation was 240 micromolar, the affinity of UDP-glucuronate for xyloglucan formation (31 micromolar) was similar to that of UDP-xylose (28 micromolar), suggesting a high turnover of UDP-xylose. The biosynthesis of UDP-xylose from UDP-glucuronate probably occurs in Golgi membranes, where xyloglucan subsequently forms from UDP-xylose and UDP-glucose.

Journal Article↗
PubMed1988

Digestibility of pentose sugars and uronic acids and their effect on chick weight gain and caecal size.

1. In the first experiment D-xylose, L-arabinose, D-galacturonic and D-glucuronic acids were fed ad libitum to young chicks for 2 weeks at 200 g/kg of diet and weight gains and food consumption were recorded. 2. L-arabinose and D-xylose did not depress food consumption in the first week but prolonged feeding caused food consumption to decrease and weight gain to be adversely affected. 3. D-galacturonic acid and D-glucuronic acid caused severe growth retardation as early as the first week of feeding, primarily because of voluntary starvation. 4. Apparent metabolisable energy values for the diets were obtained when chicks were 19 to 21 d of age and were 14.04 +/- 0.52, 12.03 +/- 0.61, 11.77 +/- 1.21, 11.68 +/- 0.34 and 11.66 +/- 0.45 KJ/g for the basal diet with glucose, xylose, arabinose, galacturonic and glucuronic acids respectively. 5. True metabolisable energy values for the diets were obtained from adult cockerels and were 15.07 +/- 0.16, 13.45 +/- 0.16, 13.12 +/- 0.37, 12.29 +/- 0.26 and 12.69 +/- 0.23 KJ/g for basal diet with glucose, xylose, arabinose, galacturonic and glucuronic acids respectively. 6. In the second experiment D-galactose, D-xylose, L-arabinose, D-galacturonic and D-glucuronic acid were fed ad libitum to young chicks for 3 weeks at 50 g/kg of diet and weight gains and food consumption were recorded. 7. Chicks grew and ate well on all diets. 8. The digestibilities of sugars and uronic acids were obtained by measurement of these constituents in diets and digesta using titanium dioxide as a marker. The digestibilities were 1.000 +/- 0.0, 0.997 +/- 0.002, 0.936 +/- 0.041, 0.628 +/- 0.103, 0.588 +/- 0.059, and 0.645 +/- 0.089 for D-glucose, D-galactose, D-xylose, L-arabinose, D-galacturonic and D-glucuronic acids respectively. 9. Both at 200 and 50 g/kg dietary inclusion there was noticeable caecal fermentation from L-arabinose, D-galacturonic and D-glucuronic acid. Only at 200 g/kg dietary inclusion did D-xylose produce significant evidence of caecal fermentation.

Animals↗
PubMed1983

Aldohexuronate transport system in Erwinia carotovora.

The biochemical and physiological aspects of hexuronate transport in Erwinia carotovora were studied to approach the genetic regulation of the hexuronate degradative pathway in this bacterial species. An active transport system for glucuronate and galacturonate uptake exists in E. carotovora. The glucuronate entry reaction displayed saturation kinetics with an apparent Km of 0.05 mM (at 25 degrees C; pH 7). Galacturonate appeared to be a competitive inhibitor of glucuronate uptake with a Ki of 0.1 mM. Glucuronate permeation was not induced by glucuronate itself in wild-type strains. Galacturonate induced the uptake of glucuronate (about fivefold). The induced synthesis of the transport system was sensitive to catabolite repression by glucose. Mutants able to grow on glucuronate as the sole carbon source showed constitutive synthesis of the hexuronate transport system.

2,4-Dinitrophenol↗
PubMed1997

Bacteroides J-37, a human intestinal bacterium, produces alpha-glucuronidase.

beta-Glucuronidases of mammalian tissues metabolized glycyrrhizin (18 beta-glycyrrhetinic acid, beta-D-glucuronyl alpha-D-glucuronic acid, GL) to glycyrrhetinic acid (GA) via 18 beta-glycyrrhetinic acid alpha-D-glucuronic acid (GAMG); they hydrolyzed beta-glucuronic acid conjugates better than alpha-glucuronic acid conjugates. However, human intestinal bacteria directly metabolized GL to GA, and minorly to GA via GAMG. Bacteroides J-37, isolated from human intestinal bacteria, transformed GL or GAMG to GA, but not baicalin; it produced alpha-glucuroniase, which hydrolyzed the alpha-linkage of glucuronic acid conjugates. alpha-Glucuronidase of Bacteroides J-37 hydrolyzed alpha-glucuronic acid conjugates better than beta-glucuronic acid conjugates. beta-Glucuronidase from E. coli, a human intestinal bacterium, hydrolyzed baicalin to baicalein, but did not transform GL.

Animals↗