Aminotransferases

Aminotransferases are widely distributed enzymes in nature, participating in a number of metabolic pathways (7-11). They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor. In effect, this statement implies the reductive animation of a keto moiety, although a redox process is not directly involved. 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

However, this is obviously not always the case, a counter example being the transformation of P-tetralone (23) to (S)—anunotetralin (24), with 2-amino butane serving as the amine donor. The of an (S)-selective aminotransaminase leads to a value of only 

A more recent study focused on the directed evolution of the co-transaminase from Vibrio fiuvialis JS17, specifically with the aim to eliminate product inhibition by aliphatic ketones while maintaining high enantioselectivity. This was achieved by screening 85 000 clones produced by epPCR [72]. 

In some bacteria and plants, however, the formation of L-glutamate is catalysed by L-glutamate synthetase, in a somewhat wasteful process in which ammonia is produced at the active site of the enzyme from L-glutamine  

The a-amino function of other L-amino acids is then produced from the corresponding a-oxo-acids through a transamination process [30,31] in which L-glutamate is almost without exception the amino donor  

When the reaction occurs as shown in Eqn. 3, the enzyme nomenclature system currently in vogue uses only the name of the unique amino acid to specify an aminotransferase, e.g. aspartate aminotransferase, alanine aminotransferase, etc. etc. 

Aspartate aminotransferase has been available in a highly purified state, and in abundance from several sources [31] for more than 2 decades. The primary amino acid sequence [32] of the enzyme as well as its tertiary structure at 2.8 A resolution [33] is now known. It is therefore not surprising that our current view on the mechanism of action of pyridoxal-P-dependent reactions in general and aminotransferases in particular is primarily based on the extensive physicochemical studies which have been performed on aspartate aminotransferase [34], 

The studies culminating in the proposal of the mechanism of action of aminotransferases as outlined in Fig. 10, include the early observation that the transformation occurs without intermediate formation of ammonia [35] and with the acquisition of one proton from the solvent which is located at C in the new amino acid [36]. It was also shown that the 8-hydrogen of the amino acid is not a participant [37-40]. 

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Percutaneous Hver biopsy after each 1.5 g of total accumulated methotrexate dosage to detect hepatic fibrosis or cirrhosis not rehably predicted by semm aminotransferase tests are recommended (1,50). Concurrent use of NSAIDs may increase toxicity of methotrexate, although toxicity may be avoided if the dmgs are separated by 12 h. 

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

Serious hepatotoxicity of tacrine has been documented. More recent data suggest, however, that this toxicity can be reduced by carehiUy monitoring semm alanine aminotransferase levels (125). The side effects of tacrine also include gastrointestinal disturbances and emesis, and alternative AChE therapies are being advanced. Velnacrine (20), a metaboUte of tacrine, was expected to have reduced hepatotoxicity. However, its limited efficacy and side-effect profile, which includes dmg-related hematological changes, caused it to be dropped from further development. 

Deamination, Transamination. Two kiads of deamination that have been observed are hydrolytic, eg, the conversion of L-tyrosiae to 4-hydroxyphenyUactic acid ia 90% yield (86), and oxidative (12,87,88), eg, isoguanine to xanthine and formycia A to formycia B. Transaminases have been developed as biocatalysts for the synthetic production of chiral amines and the resolution of racemic amines (89). The reaction possibiUties are illustrated for the stereospecific synthesis of (T)-a-phenylethylamine [98-84-0] (ee of 99%) (40) from (41) by an (5)-aminotransferase or by the resolution of the racemic amine (42) by an (R)-aminotransferase. 

In another procedure, D-amino acid oxidase (52) is usehil to produce L-amino acids from DL-amino acids. a-Ketocarboxylic acids which are formed by the action of enzymes on D-amino acids, are aminated to form L-amino acids by coupling through the action of amino acid aminotransferases (53). 

Another class of therapeutic agents is used for the treatment of certain genetic diseases or other enzymatic disorders caused by the dysfunction or absence of one particular enzyme. This often leads to an unwanted accumulation or imbalance of metaboUtes in the organism. Eor example, some anticonvulsive agents are inhibitors for y-aminobutyric acid aminotransferase [9037-67-6]. An imbalance of two neurotransmitters, glutamate and y-aminobutyric acid, is responsible for the symptoms. Inhibition of the enzyme leads to an increase of its substrate y-aminobutyric acid, decreasing the imbalance and subsequently relieving the symptoms of the disease. 

Carbonic anhydrase Pyridoxal phosphate (PLP) Amino groups Aspartate aminotransferase... 

Aminotransferases Show Double-Displacement Catalytic Mechanisms... 

One class of enzymes that follow a ping-pong-type mechanism are aminotransferases (previously known as transaminases). These enzymes catalyze the transfer of an amino group from an amino acid to an a-keto acid. The products are a new amino acid and the keto acid corresponding to the carbon skeleton of the amino donor ... 

FIGURE 14.22 Glutamate aspartate aminotransferase, an enzyme conforming to a double-displacement bisnbstrate mechanism. Glutamate aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme. The pyridoxal serves as the —NH, acceptor from glntamate to form pyridoxamine. Pyridoxamine is then the amino donor to oxaloacetate to form asparate and regenerate the pyridoxal coenzyme form. (The pyridoxamine enzyme is the E form.)... 

FIGURE 14.26 (a) Antigen used to create an abzyme with aminotransferase activity, (b) Aminotransferase reaction catalyzed by the abzyme. 

Most amino acids lose their nitrogen atom by a transamination reaction in which the -NH2 group of the amino acid changes places with the keto group of ct-ketoglutarate. The products are a new a-keto acid plus glutamate. The overall process occurs in two parts, is catalyzed by aminotransferase enzymes, and involves participation of the coenzyme pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin UJ. Different aminotransferases differ in their specificity for amino acids, but the mechanism remains the same. 

The mechanism of the first part of transamination is shown in Figure 29.14. The process begins with reaction between the a-amino acid and pyridoxal phosphate, which is covalently bonded to the aminotransferase by an iminc linkage between the side-chain -NTI2 group of a lysine residue and the PLP aldehyde group. Deprotonation/reprotonation of the PLP-amino acid imine in steps 2 and 3 effects tautomerization of the imine C=N bond, and hydrolysis of the tautomerized imine in step 4 gives an -keto acid plus pyridoxamine... 

Glutamine then reacts with a-oxoglutarate to form 2 molecules of glutamic add. This reaction is catalysed by glutamate oxoglutamate aminotransferase (GOGAT) and can be written as ... 

The activity was generally higher in cells grown in a cultivation medium containing D- or L-phenylalanine, mis confirmed the inducible nature of the phenylalanine aminotransferase. 

Practical conversion from ACA to L-phenylalanine was best achieved when acylase and aminotransferase activities were equal. This could be achieved by using 10 gram B. sphaericus and 25 gram P. denitrificam per litre medium. 

Antithyroid drags have several side effects. The most frequent side effects are maculopapular rashes, pruritus, urticaria, fever, arthralgia and swelling of the joints. They occur in 1-5% of patients [1, 2]. Loss of scalp hair, gastrointestinal problems, elevations of bone isoenzyme of alkaline phosphatase and abnormalities of taste and smell are less common. The incidence of all these untoward reactions is similar with MMI and PTU. Side effects of MMI are dose-related, whereas those of PTU are less clearly related to dose [1]. PTU may cause slight transient increases of serum aminotransferase and y-glutamyl transpeptidase concentrations but also severe hq atotoxicity whereas methimazole or carbimazole can be associated with cholestasis. The side... 

ALT, alanine aminotransferase ASC, apoptosis-associated speck-like protei containing a CARD AST, aspartate aminotransferase CARD, caspase activation and recruitment domains CD, Crohn s disease COP, CARD-only protein DD, death domain DED, death effector domains DIABLO, direct LAP-binding protein with low pi... 

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

The most common adverse reactions seen with celecoxib include dyspepsia, abdominal pain, diarrhea, nausea, and headache Like other NSAIDs, celecoxib may compromise renal function. Elevation of aminotransferase levels also occurs. 

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