Phenolic acids release from esters

Dabrowski and Sosulski, 1984 Shahidi and Naczk, 1989 Wanasundara and Shahidi, 1994a). Kozlowska et al. (1983) reported the presence of free phenolic acids (22.4 mg/100 g) in the methanolic extracts of defatted flour, but Dabrowski and Sosulski (1984) did not detect any free phenolic acids in the meal, perhaps due to the use of tetrahydro-furan for extraction. Phenolic acids released from soluble esters constituted the largest fraction (55—89%) of the total content of phenolic acids. Furthermore, Dabrowski and Sosulski (1984) reported that ferulic acid was the predominant phenolic acid in the soluble esters and insoluble residues of flaxseed. 

Additional BHT-derived 5-LO inhibitors bear heteroatom-linked 4-substituents. Searle s SC-45662 (50) was selective (25 1) for 5-LO over CO in cRBL (3.7 /iM) and in A23187-stimulated RBL-1 cells (7.1 yuM) [146]. Besides NSAID-like activity in RAA (down to 10 mg/kg p.o.), SC-45662 also inhibited GPB (ED30 16.7 mg/kg p.o.), and LTB4 release from ulcerative colitis rectal mucosal biopsy samples was decreased [147]. Several patents have described similar compounds where the alkyl substituent on sulphur is varied quite widely [148 151]. Oxidation of the distal sulphur was consistent with activity, while replacement of this sulphur with oxygen gave reduced potency. Simple alkyl groups, alkylene-linked esters and amides, and disulphide-linked alkanoic esters were also active in cRBL with similar potency free carboxylic acids were somewhat less potent. Oxidation of the sulphur attached to the phenolic ring destroyed the activity. 

PHENOLIC ACIDS BIOAVAILABILITY Release of Phenolic Acids from Esters... 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

Characterization of Humic Acids, Similar compounds are also released from soil HA and Figure 3 shows the chromatograms of the compounds released after pyrolysis in the presence of TMAH for two selected HA (77). High proportions of long-chain fatty acid methyl esters, as well as phenolic derivatives and aromatic acid methyl esters, were released. The released compounds might represent structural components of the humic macromolecule, as also suggested by different authors... 

Odor compounds may also be released from the plastic materials used in cars. The variety of plastics and possible chemical compounds is broad, which makes the identification of odor causing compounds an extremely comphcated task. An effective and rapid screening of VOCs and semi-VOCs from materials used in automobiles was developed by utihzing the SPME technique [28]. The low molecular weight compoimds extracted from five different automobile materials included different benzene derivatives, aldehydes, esters, biphenyls, phthalates, butylated hydroxytoluene, phenols, alcohols, styrene, triethylene-diamine, carboxylic acids and ketones. A considerable munber of VOCs and semi-VOCs were detected, indicating that more attention should be paid to the selection of materials and additives for automotive parts. 

Several feruloyl esterases have been purified and characterized (Table 1). However, comparison of their properties is difficult as the range of natural and synthetic substrates used to characterize these enzymes is diverse and the enzyme assays are not unifomi. The substrates range in size and complexity from small, soluble esters such as feruloylated oligosaccharides isolated from plant cell walls and phenolic acid methyl esters or synthetic feruloylated arabinosides to larger, more complex and often less soluble substrates such as feruloylated polymeric plant cell wall fractions (28). The only criterion used in all cases is the release of free ferulic acid or another hydroxycinnamic acid by hydrolysis of an ester bond. Specificity, as defined by Ae rate of catalysis (kca divided by the Michaelis constant gives the best indication of preferred substrates. However, hydrolysis of polymeric substrates is more complicated since not all of the esterified substituents are chemically equal, and effects such as decreased solubility and steric hindrance further complicates any results obtained. Therefore, these data should not be extrapolated to obtain kinetic constants. 

Cell walls can be rapidly isolated from plant tissues by a modification of the method of Van Soest and Wine which involves extraction with neutral detergent. Most investigators have treated isolated cell walls with alkali to release phenolic acids as their sodium salts. The acids can also be released as their carbohydrate esters by degrading the walls with a commercial cellulase preparation from fungal sources (Basidiomycete, Oxyporous spp.) having... 

TJesticides derived from chlorinated phenols (Table I) are among the most prominent of those currently in worldwide use. Several major herbicides have been applied in large quantities in subtropical locations. Cahfornia used more than 1,200,000 pounds of 2,4-dichlorophenoxyacetic acid (2,4-D) and its derivatives in 1970 (I) Hawaii consumed some 465,000 pounds of pentachlorophenol (PCP) in 1968 (2), and the amount of combined butyl esters of 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) released in one area of Cambodia during two months of 1969 was estimated to exceed 77,000 pounds (3). 

The chemical diversity of carboxylic acid esters (R-CO-O-R ) originates in both moieties, i.e., the acyl group (R-CO-) and the alkoxy or aryloxy group (-OR7). Thus, the acyl group can be made up of aliphatic or aromatic carboxylic acids, carbamic acids, or carbonic acids, and the -OR7 moiety may be derived from an alcohol, an enol, or a phenol. When a thiol is involved, a thioester R-CO-S-R7 is formed. The model substrates to be discussed in Sect. 7.3 will, thus, be classified according to the chemical nature of the -OR7 (or -SR7) moiety, i.e., the alcohol, phenol, or thiol that is the first product to be released during the hydrolase-catalyzed reaction (see Chapt. 3). Diesters represent substrates of special interest and will be presented separately. 

There is also evidence that at least some of the phenolic aldehydes and dehydrodiferulic acid (Figure 1) are linked covalently to cell wall polysaccharides. When ryegrass cell walls were treated with cellulase, the aldehydes and the acid were released as water-soluble carbohydrate-aromatic compounds from which the aromatics were released by cold sodium hydroxide treatment (6,7). This suggests that these compounds are either ether-linked or, in the case of the acid, ester-linked to the polysaccharides. 

Cleavage conditions for alkyl benzyl ethers prepared from acid-labile benzyl alcohols are similar to those for the corresponding benzyl esters (Table 3.30). Aryl benzyl ethers, however, are generally cleaved more easily by acidolysis than esters or alkyl ethers. Phenols etherified with hydroxymethyl polystyrene, for instance, can even be released by treatment with TFA (Entry 1, Table 3.31). It has also been shown that Wang resin derived phenyl ethers are less stable than Wang resin derived esters towards refluxing acetic acid [29]. Alternatively, boron tribromide may be used to cleave aryl ethers from hydroxymethyl polystyrene [573],... 

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