Conjugation reactions—

The main Phase II metabolite of valproic acid (29), a broad spectrum anticonvulsant, is the acyl glucuronide l-0-valproyl-/3-D-glucopyranuronic acid (30). This metabolite was prepared in carbon-14 and tritium-labeled forms enzymatically in one step in around 52% yields using UDP-glucuronosyltransferase . The positions of labels were not specified. In contrast, the chemical synthesis starting from valproic acid and 2,3,4,6-tetrabenzyl-D-glucopyranose required three steps and provided the required /3-anomer in only 6.8% yield. 

Reaction conditions UDP-glucuronic acid, UDP-glucuronosyltransferase, glycerine-NaOH buffer, pH 8.0 37 °C, 48 h 

Use of multi-enzyme systems Combining two or more purified enzymes in the same pot can facilitate the preparation of more stmcturally complex molecules. One example of this is the bi-enzymatic system of PG-synthase and PGH-converting enzyme isolated from various mammalian tissues. This mixture was used to converted various tritium-labeled polyunsaturated fatty acids to prostaglandins D, E and F and thromboxane B at high specific activities and yields of 20-80%.  

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This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Conidine — see 1-A2abicyclo[4.2.0]octane Conjugation reactions nitrogen heterocycles at carbon, 1, 239 at nitrogen, 1, 234-238 Conjunction nomenclature, 1, 37 Contagious bovine pleuropneumonia heterocyclic compounds as, 1, 205 A Convention... 

Acetylation is another conjugation reaction, but it differs from the foregoing examples in that the products tend to be less polar than the substrates. 

CONJUGATION REACTIONS PREPARE XENOBIOTICS FOR EXCRETION IN PHASE 2 OF THEIR METABOLISM... 

Bagotskii VS, Tarasevich MR, Fihnovskii VY. 1969. Calculation of the kinetic parameters of conjugated reactions of oxygen and hydrogen peroxide. Elektrokhimiya 5 1218. 

Testa B (2006) Principles of drug metabolism 2 hydrolysis and conjugation reactions. In Taylor JB, Triggle DJ (eds) Comprehensive medicinal chemistry II. Elsevier, Oxford, Sect 5.06... 

In BA metabolism, the procarcinogenic BA trans-3.4-dihydrodiol (26) constitutes 1.5-4% of all the metabolites formed by rat liver microsomes (27) and a major component of the free dihydrodiols formed by mouse skin maintained in short-term organ culture (28). In this system (28). the noncarcinogenic dihydrodiols may be preferentially removed by conjugation reactions to yield water soluble products. 

The metabolic formation of N-sulfonyloxy-N-acetyl-2-aminofluorene (N-sulfonyloxy-AAF) and its observed electrophilic reactivity, provided the first evidence for the importance of enzymatic conjugation reactions in chemical carcinogenesis (23,24). This reaction was shown to be catalyzed by PAPS-dependent sulfotrans-ferases that are located predominantly in liver cytosol and has been subsequently demonstrated for N-hydroxy arylamide metabolites of several other carcinogens, including N-acetyl-4-aminobiphenyl (AABP), benzidine, N-acetyl-2-aminophenanthrene and phenacetin. 

Amine functionalities also may be created on polysaccharides (Section 4.3, this chapter). The reducing ends of carbohydrate molecules (or generated aldehydes) may be reacted with small diamine compounds to yield short alkylamine spacers that can be used for subsequent conjugation reactions. Hydrazide groups may be similarly created using f z s-hydrazide compounds (Sections 4.5 and 4.6, this chapter). 

Purify the thiolated protein from excess DTT by dialysis or gel filtration using 50 mM sodium phosphate, 0.15 M NaCl, ImM EDTA, pH 7.2. The modified protein should be used immediately in a conjugation reaction to prevent sulfhydryl oxidation and formation of disulfide crosslinks. 

Figure 1.70 AMBH is a hydrazide-containing compound that reacts with carbonyl groups to form hydrazone bonds. The free thiol can be used for subsequent conjugation reactions.

This simple strategy can be used to add amine residues to polysaccharide molecules after formation of aldehydes by periodate or enzymatic oxidation (Section 4.4, this chapter). Thus, glycoconjugates or carbohydrate polymers such as dextran may be derivatized to contain amines for further conjugation reactions. 

The formation of an aldehyde group on a macromolecule can produce an extremely useful derivative for subsequent modification or conjugation reactions. In their native state, proteins, peptides, nucleic acids, and oligonucleotides contain no naturally occurring aldehyde residues. There are no aldehydes on amino acid side chains, none introduced by post-translational modifications, and no formyl groups on any of the bases or sugars of DNA and RNA. To create reactive aldehydes at specific locations within these molecules opens the possibility of directing modification reactions toward discrete sites within the macromolecule. 

Figure 1.102 The reaction of sodium periodate with sugar residues can produce aldehydes for conjugation reactions.

Figure 1.105 Glutaraldehyde can undergo complex reactions with amine groups, resulting in aldehyde-containing derivatives that can be used in conjugation reactions.

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