Deacetylation, enzymatic

Another method of deacetylation (enzymatic bioconversion) makes use of enzyme, chitin deacetylase foimd in several fungi and in some insect species [15-17]. It acts by catalyzing deacetylation of N-acetylglucosamine residues, resulting in conversion of chitin to chitosan. 

The role of N-acetoxy arylamides as metabolically formed ultimate carcinogens jji vivo also appears to be limited. Their enzymatic formation via peroxidation of N-hydroxy arylamides can be excluded since tissues containing high levels of peroxidases such as the rat mammary gland (83) and the dog urinary bladder (84) do not form acetylated carcinogen-DNA adducts in vivo (63). Their non-enzymatic formation by reaction of acetyl coenzyme A with N-hydroxy arylamides (6 ) cannot be excluded however, even if formed, their direct reaction with cellular DNA appears unlikely as treatment of cultured cells with synthetic N-acetoxy AAF (85,86) results primarily in deacetylated arylamine-DNA adducts, apparently due to rapid N-deacetylation to form the reactive N-acetoxy arylamine (V). 

Prolonged residence in the intestine or urinary bladder lumen could allow time for significant reaction with tissue components however, N-glucuronyloxy-AAF was only weakly carcinogenic at local subcutaneous sites of application (89). Enzymatic deacetylation to N-glucuronyloxy-AF has been detected in hepatic tissue but this activity in different species does not correlate with their relative susceptibility to AAF hepatocarcinogenesis (94). On the other hand, the alkaline pH-induced conversion to a reactive derivative may play an important role in urinary bladder carcinogenesis (87) by AAF and other arylamides in those species or individuals where normal urine pH is alkaline (e.g. normal rabbit urine pH is 8.5-9.0). 

Commonly, in vitro determination of HDAC activity is a manual assay utilizing a coupled two-step process, including enzymatic deacetylation of a substrate followed by reaction termination and readout [10]. Assays utilize nuclear extracts and substrates containing labeled (radioactive or fluorescent) acetylated histones. For the isotope-based assays, the enzymes are incubated with acetate-radiolabled histones prepared from chicken reticulocytes or chemically [ Hjacetylated peptide substrates, and the enzymatic activity is determined by liquid scintillation counting [11]. Alternatively, histones may be obtained from cells following treatment with [ H]acetyl-CoA [12]. The caveats of these approaches include the variability of prelabeled acetylated core histones within preparations, potential high costs, their labor-intensive nature and the presence of radioactive waste. 

Fig.l The dynamic equilibrium between acetylation and deacetylation of lysine residues of the histones is controlled by the opposing enzymatic activities of HATs and HDACs. The acetylation status determines whether a lysine residue is either neutral (acetylated) or positively charged (deacetylated). The consequent changes in the internucleosomal interactions and condensation status of chromosomal domains govern the transcriptional competence of DNA ( Diane Bruyninckx)... 

As a general mechanism, the degradation of PVA starts outside the cells via enzymatic attack on the polymer. The resulting products are a mixture of acetoxy hydroxy and hydroxy fatty acids. Upon intracellular enzymatic deacetylation, hydroxy fatty acids are generated that can be further metabolised via the classical p-oxidation pathway and TCA cycle. 

As enzymatic oxidative transformation of the PVA polymer can act as a multiple simultaneous event on the polymer with concurrent chain fission by the appropriate enzymes, the polymer can be broken down into small oligomers that can be channelled into the primary metabolism. This picture is not complete because PVA is usually more or less acetylated. The DH is a pivotal factor in almost every aspect of PVA application. Surprisingly there are very few data dealing with the enzymes involved in the deacetylation of not fuUy hydrolysed PVA polymer. In technical processes, esterase enzymes are widely applied to deal with PVAc structures. A good example is from the pulp and paper industry [85], where PVAc, a component of stickies , is hydrolysed to the less sticky PVA. Esterases from natural sources are known to accept the acetyl residues on the polymer as substrate but little detailed knowledge exists about the identity of acetyl esterases in the PVA degradative environment [86]. 

Chemical synthesis of racemates and subsequent resolution via crystallization of diastereomeric salts is employed in the preparation of rf-biotin and tocopherol (vitamins), dexchlorpheniramine (antihistaminic), levomepromazine (neuroleptic), levorphanol (analgesic), and naproxen (antiphlogistic), to note some examples4, threo-2-Amino-1 -(4-nitro-phenyl)-l,3-propanediol, an intermediate in the production of chloramphenicol, is resolved by crystallization with entrainment or via crystallization of the salt with camphorsulfonic acid4. Enzymatic resolutions are increasingly employed, normally via deacetylation of racemic acetates. This method has recently been used in the synthesis of carbacyclin derivatives5. 

Scheme 10.8. Enzymatic acetylation of diols and deacetylation of diacetates [30-34],...

An enzymatic method for resolution of the racemic cis-2,8-diazabicyclo[4.3.0[-nonane has also been described, in which reaction of the S,S-enantiomer with ethyl acetate in the presence of lipase produces the S,S-diacetyl derivative under the reaction conditions and the R, K-eriari liorner is only reacted to form the monoacetyl derivative. The components can be readily separated from the resulting reaction mixture, and then deacetylated [141]. 

The most important derivative of chitin is chitosan obtained by partial deacetylation of chitin in the solid state under alkaline conditions or by enzymatic hydrolysis in the presence of a chitin deacetylase. The ratio of 2-acetamido-2-deoxy-D-glucopyranose to 2-amino-2-deoxy-D-glucopyra-nose moieties determines the identity of the product, that is, chitin or chitosan [9]. The published methods used for the production of chitosan from chitin are summarized in Table 2.3. 

Chitin is a stable compoimd, incompatible with oxidizing agents [59]. In the solid state imder alkaline condition (e.g., NaOH, KOH, heat at about 120 °C) or by enzymatic hydrolysis in the presence of a chitin deacetylase, it hydrolyses to form the deacetylated degradation product chitosan [6,7,10,11]. It was found that the presence of urea in basic media and at low temperature (—20 °C) had little effect on chitin structure and that urea is of benefit to the stability of chitin solution [38]. 

Sir2 family of NAD+-dependent protein deacetylases that is involved in a caloric restriction-dependent life span extension, has a strong preference for histone H4K16Ac in its deacetylation activity. Mouse embryonic fibroblasts (MEFs) deficient for SirT2 show higher levels of H4K16Ac in mitosis when compared with the normal levels exhibited by SirTl-deficient MEFs (21). The enzymatic conversion of H4K16Ac to its deacetylated form may be pivotal to the formation of condensed chromatin. 

The greater the degree of NBS oxidation of the enzyme the slower was the rate with which its acetyl derivative parted with the acetate residue. The results are shown in Fig. 28. Interestingly enough the acetyl derivative of unoxidized chymotrypsin lost its acetate residue at a much slower rate than the rate of appearance of maximum enzymatic activity. The half-time of deacetylation was about 3 min, whereas maximal activity with the acetylchymotrypsin was obtained about 1.0-1.5 min after its addition to substrate at pH 8.0. 

Biosynthesis of GPI anchors starts with the core structure assembly by sequential addition of UDP-GlcNAc (followed by iV-deacetylation), dolichol-phosphate-mannose, and phospho-ethanolamine to phosphatidylinositol and culminates in the en bloc transfer to protein shortly after the protein is synthesized. However, the biosynthetic pathways can differ strikingly between different organisms with respect to specific modifications and fatty acid remodeling occurring after completion of the core glycan. This also applies for the point when certain modifications are introduced, e. g. before or after the transfer of the GPI-moiety to the protein. GPI anchors can be cleaved at defined positions by an array of enzymatic and chemical methods, respectively (O Fig. 5). Thus, it becomes possible to identify GPI-anchored proteins and, moreover, analyze the structure and biosynthesis of GPI anchors [103]. 

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