Xanthine

About: Xanthine is a research topic. Over the lifetime, 4046 publications have been published within this topic receiving 129820 citations. The topic is also known as: Xanthine.

Effects of hyperglycemia and oxidative stress on the glutamate transporters GLAST and system xc- in mouse retinal Müller glial cells.

Abstract: Elevated glutamate levels have been reported in humans with diabetic retinopathy. Retinal Muller glial cells regulate glutamate levels via the GLAST transporter and system x(c)(-) (cystine-glutamate exchanger). We have investigated whether transporter function and gene and/or protein expression are altered in mouse Muller cells cultured under conditions of hyperglycemia or oxidative stress (two factors implicated in diabetic retinopathy). Cells were subjected to hyperglycemic conditions (35 mM glucose) over an 8-day period or to oxidative stress conditions (induced by exposure to various concentrations of xanthine:xanthine oxidase) for 6 h. The Na(+)-dependent and -independent uptake of [(3)H] glutamate was assessed as a measure of GLAST and system x(c)(-) function, respectively. Hyperglycemia did not alter the uptake of [(3)H] glutamate by GLAST or system x(c)(-); neither gene nor protein expression decreased. Oxidative stress (70:14 or 100:20 microM xanthine:mU/ml xanthine oxidase) decreased GLAST activity by approximately 10% but increased system x(c)(-) activity by 43% and 89%, respectively. Kinetic analysis showed an oxidative-stress-induced change in V(max), but not K(m). Oxidative stress caused a 2.4-fold increase in mRNA encoding xCT, the unique component of system x(c)(-). Of the two isoforms of xCT (40 and 50 kDa), oxidative stress induced a 3.6-fold increase in the 40-kDa form localized to the plasma membrane. This is the first report of the differential expression and localization of xCT isoforms as caused by cellular stress. Increased system x(c)(-) activity in Muller cells subjected to conditions associated with diabetic retinopathy may be beneficial, as this exchanger is important for the synthesis of the antioxidant glutathione.

Biosynthesis of caffeine by tea-leaf extracts. Enzymic formation of theobromine from 7-methylxanthine and of caffeine from theobromine.

Abstract: 1. Extracts prepared from tea leaves with Polyclar AT (insoluble polyvinylpyrrolidine) contained two methyltransferase activities catalysing the transfer of methyl groups from S-adenosylmethionine to 7-methylxanthine, producing theobromine, and to theobromine, producing caffeine. 2. The methyltransferases exhibited the same pH optimum (8.4) and a similar pattern of effects by metal ions, thiol inhibitors and metal-chelating reagents, both for theobromine and caffeine synthesis. Mg2+, Mn2+ and Ca2+ slightly stimulated enzyme activity but they were not essential. Paraxanthine was shown to be most active among methylxanthines, as the methyl acceptor. However, the formation of paraxanthine from 1-methylxanthine was very low and that from 7-methylxanthine was nil, suggesting that the synthesis of caffeine from paraxanthine is of little importance in intact plants. Xanthine, xanthosine, XMP and hypoxanthine were all inactive as methyl acceptors, whereas [2(-14)C]xanthine and [8(-14)C]hypoxanthine were catabolized to allantoin and urea by tea-leaf extracts. The apparent Km values are as follows: 7-methylxanthine, 1.0 times 10(-14)M; theobromine, 1.0 times 10(-3)M; paraxanthine, 0.2 times 10(-3)M; S-adenosylmethionine, 0.25 times 10(-4)M (with each of the three substrates). 3. The results suggest that the pathway for caffeine biosynthesis is as follows: 7-methylxanthine leads to theobromine leads to caffeine. In contrast, it is suggested that theophylline is synthesized from 1-methylxanthine. The methyl groups of the purine ring of caffeine are all derived directly from the methyl group of S-adenosylmethionine. Little is known about the pathways leading to the formation of 7-methylxanthine. 4. A good correlation between caffeine synthesis and shoot formation or growth of tea seedlings was shown, suggesting that the methylating systems in caffeine synthesis are closely associated with purine nucleotide and nucleic acid metabolism in tea plants.

Degradation of nucleic acid derivatives by rumen bacteria in vitro.

Abstract: 1. Purine or pyrimidine bases, nucleosides or nucleotides were incubated, in vitro, with whole rumen contents, or with a cell-free filtrate prepared from rumen contents of calves fed on diets of flaked maize and hay.2. During incubations with whole rumen contents, all derivatives bearing a side amino group were deaminated to varying extents.3. All nucleotides and nucleosides were rapidly degraded to the parent base or its desamino counterpart.4. Evidence suggests that guanosine was degraded to guanine before deamination occurred whereas deamination preceded ribosidic cleavage for adenosine.5. Xanthine, uracil, hypoxanthine and thymine resisted further degradation to varying degrees.6. Little or no breakdown was observed when any derivative was incubated, in vitro, with cell-free preparations.

[3H]xanthine amine congener of 1,3-dipropyl-8-phenylxanthine: an antagonist radioligand for adenosine receptors.

Abstract: An amine-functionalized derivative of 1,3-dipropyl-8-phenylxanthine has been prepared in tritiated form as a xanthine amine congener ([3H]XAC) for use as an antagonist radioligand for adenosine receptors. [3H]XAC has higher receptor affinity, higher specific activity, lower nonspecific membrane binding, and more favorable hydrophilicity than 1,3-diethyl-8-[3H]phenylxanthine, the xanthine commonly used for adenosine receptor binding. In rat cerebral cortical membranes, [3H]XAC exhibits saturable, specific binding with a Kd of 1.23 nM and a Bmax of 580 fmol/mg of protein at 37 degrees C. N6-(R-Phenylisopropyl)adenosine is a more potent inhibitor of [3H]XAC binding than is 5'-N-ethylcarboxamidoadenosine, indicating that binding is to an A1-adenosine receptor. In the absence of GTP, the inhibition curves for adenosine agonists versus [3H]XAC binding are biphasic, indicating that [3H]XAC is binding to low- and high-affinity agonist states of the A1 receptor. In the presence of GTP, adenosine analogs exhibit monophasic, low-affinity inhibition of binding of [3H]XAC. Inhibition of [3H]XAC binding by theophylline or by various 8-phenylxanthines is monophasic, and the potencies are commensurate with the potencies of these xanthines as adenosine receptor antagonists. The receptor sites in calf brain membranes exhibit a higher affinity (Kd = 0.17 nM) for [3H]XAC, whereas sites in guinea pig exhibit a slightly lower affinity (Kd = 3.0 nM). Densities of [3H]XAC binding sites are similar in brain membranes from all species.