Thiobarbiturate formation

The cyclizations to obtain cyclic thioureas have been performed using thiocarbonyldiimidazole.232 Reaction of methyl acetoacetate, thiourea and an aliphatic aldehyde in the presence of the zinc iodide (Znl2) was studied. Under the normal pressure, reaction has not been occurred whereas at high pressure (300 MPa) conditions 3,4-dihydropyrimidine-2-thione was obtained only in 10% yield.233 The same one-pot three-component cyclocondensation reaction in the presence of iodide (I2) provides a variety of 3,4-dihydropyrimi-dine-2-thione in high yields.234 Condensation reaction of thioureas with a,p-unsaturated ketones in the presence of the sodium methoxide in methanol affords 3,4-dihydropyrimidine-2-thione derivatives.235,236 Acylation of N,N -disubstituted thioureas with methyl malonyl chloride followed by base-catalysed cyclization leads in the formation of l,3-disubstituted-2-thiobarbituric acids (Scheme 78).237... 

Most of the structural and biochemical work related to KDO is based on the estimation of the compound or its derivatives by the periodate-thiobarbituric acid (TBA) assay in its various modifications. Indeed, KDO (see Fig. 3) was discovered9 through the formation of a characteristic, purple, TBA chromophore (Xmax 549 nm) from its 8-phosphate (2), which is the product of the condensation of D-arabinose 5-phosphate with enolpyruvate phosphate, catalyzed by 3-deoxy-8-0-phosphonooctulosonate synthetase (EC 4.1.2.16) (see Scheme 1 and Section V,2). 

The concentration of ozone taken up by the media containing linolenic acid is plotted against time after addition in Figure 8. The rate of ozone breakdown is constant (ozone uptake linear with time) for the first two min until about 0.12 ml ozone are absorbed and then the rate decreases sharply, reaching a steady-state rate of ozone uptake between 10-12 min. This first break in the curve corresponds to an ozone uptake of 0.12 ml + (24 moles/liter) = 0.005 millimoles (or 10 M). This is equivalent to 1 mole of linolenic acid added per mole ozone absorbed. Thiobarbituric acid reactant production is also plotted on the same axis. This compound (TBA reactant) probably arises by formation of a three-carbon fragment (malondialdehyde) from the ozone-induced oxidation of linolenic acid (23). The rate of TBA reactant formation is also linear for the first 2 min at which point the curve undergoes a less pronounced break. Malondialdehyde formation ceases immediately when the ozone is shut off (Scrub 1 on). An oxygen control sample produced no malondialdehyde. 

Other indices measure a secondary stage of oxidation, such as the anisidine value (ANV), pointing to formation of carbonyl compounds, capable of undergoing condensation reactions with p-anisidine, and the thiobarbituric acid reactive substance (TBARS) pointing to the presence of malondialdehyde (MDA) in particular. In biological systems, TBARS is of widespread use as a measure for the extent of oxidation damage. Another test for stability of oils to oxidation is based on the development of acidity as secondary product, for example, standards using the Rancimat equipment or a similar setup. 

Antioxidant activity was also tested in a liver microsome system. In this study, mice were treated by oral intubation (2 times/wk) with 0.2 ml olive oil alone or containing CLA (0.1 ml), linoleic acid (0.1 ml), or dl-a-tocopherol (lOmg). Four weeks after the first treatment, liver microsomes were prepared and subsequently subjected to oxidative stress using a non-enzymatic iron-dependent lipid peroxidation system. Microsomal lipid peroxidation was measured as thiobarbituric acid-reactive substance (TBARS) production using malondialdehyde as the standard. It was found that pretreatment of mice with CLA or dl-a-tocopherol significantly decreased TBARS formation in mouse liver microsomes (p < 0.05) (Sword, J. T. and M. W. Pariza, University of Wisconsin, unpublished data). 

In an effort to investigate antioxidant constituents with antiproliferative effects in rat vascular smooth muscle cells (VSMC), broussoflavan A (36) [49], broussoflavonols F (45) [50] and G (46) [51], and broussoaurone A (48) [49] were found to inhibit the Fe2+-induced thiobarbituric acid-reactive substance formation in rat brain homogenate. Furthermore, broussoflavonols F (45) and G (46) inhibited fetal calf serum-, 5-hydroxytryptamine-, or ADP-induced [3H]thymidine incorporation into rat VSMC [45]. Antioxidant activities and inhibitory effects on proliferation of rat VSMC with potent antiplatelet activities of 45 and 46 may be useful for vascular diseases and atherosclerosis [43,45]. 

In Sprague-Dawley rats treated orally with 50, 75, 100 or 150 mg/kg bw acrylonitrile, hepatic nonprotein sulfhydryl concentration was significantly decreased after 30 min (Silver et al., 1982). Twenty-four hours after administration of 150 mg/kg bw, focal liver necrosis was observed. In isolated rat hepatocytes, acrylonitrile (1 mM) treatment resulted in the formation of thiobarbituric acid-positive products (a test for malon-aldehyde) and in depletion of non-protein sulfhydryl groups, but did not affect markedly the viability of the cells (Nerudova et al., 1988). 

Kosugi, H., Kato, T., and Kikugawa, K. 1987. Formation of yellow, orange, and red pigments in the reaction of alk-2-enals with 2-thiobarbituric acid. Anal. Biochem. 165 456-464. 

Tarladgis, B.G., Pearson, A.M., and Dugan, L.R. Jr. 1964. Chemistry of the 2-thiobarbituric acid test for determination of oxidative rancidity in foods. II.-Formation of the TBA-malonaldehyde complex without acid-heat treatment. J. Sci. Food Agric. 15 602-607. 

The second enzyme in the sequence studied was KD0-8-phosphate synthase. This enzyme was purified to homogeneity (24). This enzyme catalyzes the condensation of D-arabinose-5-phosphate and PEP to yield KD0-8-phosphate and inorganic phosphate. One can assay this irreversible reaction either by measuring the formation of KD0-8-phosphate or the release of P. (2Jj). The latter is the method of choice, since a number of analogues were found to interfer with the thiobarbituric acid assay. The enzyme has an apparent K for PEP of 6 x 10 M and an apparent K for D-arabinose-5-phospftiate of 2 x 10 M. m... 

Although other natural sugars were tested as possible alternate substrates the specificity for KDO was absolute. However, the nucleotide CTP could be replaced by deoxy-CTP as a nucleotide acceptor when the reaction was carried out at both pH 8.0 and pH 9.5. Also, UTP was an effective nucleotide acceptor when the reaction was carried out at pH 9.5 but much less so when tested at pH 8.0. No other nucleotide tested acted as a substrate or inhibited the formation of the NaBH,-resistant thiobarbituric acid positive reaction. The apparent K values.determined at.pH 9.5 for CTPdeoxy-CTP and UTP were fto x 10 4 M, 3.4 x 10 M, and 8.8 x 10 M, respectively. Th apparent V values for CTP,... 

The autoxidation of polyunsaturated fatty acids (cf. Porter et al. 1981) is usually monitored by the formation of malonaldehyde using the 2-thiobarbituric acid essay. This is carried out under rather severe conditions which decomposes its precursor. This malonaldehyde-like product is obviously formed via a cycliza-tion reaction of a peroxyl radical, followed by other processes such as further cyclization and hydroperoxide formation [reactions (21)-(23)]. The resulting hydroperoxides may eliminate malonaldehyde upon a homolytic cleavage of the endoperoxidic intermediate (Pryor and Stanley 1975). 

It has been known for quite some time that in the OH reaction with DNA in the presence of 02 lesions are formed which give rise to the formation of an intensive pink color (Amax = 532 nm) upon reaction with 2-thiobarbituric acid (TBA) in acid solution (Krushinskaya and Shalnov 1967 Gutteridge 1982 Gutteridge and... 

Thiobarbituric acid 235 (Scheme 48) also reacts first at the methylene group, then at the nitrogen atoms, as in the formation of 236 and 237 (93T1457). 

TBA value Formation of malonaldehyde on oxidation of lipids and its reaction with 2-thiobarbituric acid Gray456... 

In a study on butteroil held at a temperature ranging from —10 to +50°C, oxidation rate increased with increasing temperature but the same flavor was formed on storage and the reaction sequence for flavor formation was similar at all temperatures (Hamm et al., 1968). Dunkley and Franke (1967) reported a decrease in flavor intensity and thiobarbituric acid (TBA) values in liquid milk as storage temperature was increased from 0 to 4 to 8°C. Schwartz and Parks (1974) reported that condensed milk stored at — 17°C was more susceptible to oxidized flavor development than at — 7°C. 

Change of mass, peroxide value or hydroperoxides, conjugated dienes, 2-thiobarbituric acid reactive substances, anisidine value, formation of hexanal, ethane, or pentane... 

Thiobarbituric Acid Value. The 2-thiobarbituric acid (TEA) test is a popular method for measuring sensory oxidation products. It is based on the formation of a colored complex between two molecules of TEA reagent with one molecule of malonaldehyde or TEA reactive substances (TEARS). This intensity of the pink chromogram is measured at 532 nm. 

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