Tyrosine-a-ketoglutaric transaminase

Fig. 5. Changes of liver enzyme activities after tyrosine dosage of guinea pigs tyrosine-a-ketoglutarate transaminase (- -o- -), p-hydroxyphenylpyruvate oxidase (— x—). Each point is the average of homogenates from two animals. From Knox and Goswami (Kll).

Sereni F, Kenny FT, Kretchmer N. Factors influencing the development of tyrosine-a-ketoglutarate transaminase activity in rat liver. J Biol Chem 1959 234 609-612. 

Wicks, W. D. 1968. Tyrosine-a-ketoglutarate transaminase Induction by epinephrine and adenosine 3 , 5 cyclic phosphate. Science, 160 997. 

Injection of hydrocortisone into the rat results in an approximately four fold increase in liver (but not kidney) tyrosine-a-ketoglutarate transaminase activity 374)- Corticosterone and cortisone were somewhat less effective. Interesting in this connection is the observation that injection of L-tyrosine increased the activity of tyrosine-a-ketoglutarate transaminase to about the same extent as hydrocortisone. Other amino acids had a much smaller stimulating effect which was attributed to adrenal cortical stress. In a preliminary report the addition of hydrocortisone to lymphocyte suspensions was found to inhibit glutamic-oxalacetic transaminase activity 376). 

Kenney ( 09) also has considerably purified a tyrosine-a-ketoglutarate transaminase from rat liver. Its properties are quite similar to those of the dog liver enzyme. 

Besides the mineralcorticoid effect on the electrolyte metabolism, the adrenocortical hormones also influence the metabolism of glucose by promoting glycogen formation in the liver, especially from protein. This effect is termed the glucocorticoid effect and is manifested especially by the 11/3-hydroxy compounds. Emphasizing the breakdown of protein, this effect is also called the catabolic effect. The mechanism of this hormone action seems to involve the de novo formation of more enzymes of amino acid metabolism, e.g., tyrosine a-ketoglutarate transaminase, tr5rptophan pyrrolase, etc. (cf. Enzyme Induction, Chapt. VII-7). 

Reel, J. R., Lee, K.-L., and Kenney, F. J., 1970, Regulation of tyrosine a-ketoglutarate transaminase in rat liver. VIII. Inductions by hydrocortisone and insulin in cultured hepatoma cells, J. Biol. Chem. 245 5800. 

Thompson EB, Tomkins GM, Curran JF. 1966. Induction of tyrosine alpha-ketoglutarate transaminase by steroid hormones in a newly established tissue culture cell line. Proc. Natl. Acad. Sci. USA 56 296-303... 

Tjo osine-a-ketoglutarate transaminase (210) is an inducible enzyme much like tryptophan pyrrolase (see Section XIII). The ability to increase the transaminase activity in liver by tyrosine administration depends on the integrity of adrenal cortical function. 

This enzyme [EC 2.6.1.1] (also known as transaminase A, glutamicioxaloacetic transaminase, and glutamic aspartic transaminase) catalyzes the reversible reaction of aspartate with a-ketoglutarate to produce oxaloace-tate and glutamate. Pyridoxal phosphate is a required cofactor. The enzyme has a relatively broad specificity, and tyrosine, phenylalanine, and tryptophan can all serve as substrates. 

This pyridoxal-phosphate-dependent enzyme [EC 2.6.1.5], also known as tyrosine transaminase, catalyzes the reaction of L-tyrosine with a-ketoglutarate (or, 2-oxoglutarate) to produce 4-hydroxyphenylpyruvate and L-glutamate. L-Phenylalanine can act as the substrate instead of tyrosine. In some systems, the mitochondrial enzyme may be identical with aspartate aminotransferase. 

Indolmydn.—Previous evidence on the biosynthesis of indolmycin (88) in Strepto-myces griseus cultures accords with the pathway shown in Scheme 4. The first two steps in the pathway have been carried out using cell-free extracts of 5. griseus - and recent work has led to the isolation of two enzymes which can effect these transformations. The first, tryptophan transaminase, catalysed the pyridoxal phosphate-dependent transamination of L-tryptophan, but not D-trptophan, and in common with some other microbial transaminases, a-ketoglutarate was an efficient amino-group acceptor. L-Phenylalanine, tyrosine, and 3-methyltryptophan (this compound inhibited enzyme function) also underwent transamination. 

A number of amino acids, like alanine, leucine, tyrosine, aspartic acid, cystein and arginine react with a-ketoacids and transfer their a-amino group to the a-carb-on of the a-keto acids. These reactions are catalysed by the enzyme called transaminase or aminotransferase. For example, transfer of the amino group of aspartic acid (14) to a-ketoglutaric acid (23) gives glutamic acid (16) and oxaloacetic acid (24). 

In the presence of a-ketoglutarate and pyridoxal phosphate, tyrosine loses its amino group to yield hy-droxyphenylpyruvate. The enzyme involved in that reaction—tyrosine transaminase—has been extensively purified from dog liver, and some aspects of its... 

In the same scheme, a representation of the action of tyrosine transaminase (EC 2.6.1.5) acting on tyrosine (Tyr, Y) is shown. This enzyme utilizes pyridoxal phosphate to remove toe amino group from tyrosine (Tyr, Y) and transfer it to a-ketoglutarate (2-oxoglutarate) with formation of glutamate (Glu, E) from the latter and 4-hydroxyphenylpyruvate from the former. Then, 4-hydroxyphenylpyruvate decarboxylase (EC 4.1.1.80), which appears to require thiamine diphosphate and... 

The first step in tyrosine oxidation is a transamination to form p-hydroxyphenylpyruvic acid. Several groups of investigators independently showed a dependence of tyrosine oxidation on the presence of a keto acid. Knox and LeMay-Knox showed that a-ketoglutarate is a specific partner in the transamination and that pyridoxal phosphate is a cofactor in this reaction. Partial resolution of the transaminase allowed a demonstration of parallel restoration of transaminase activity and over-all tyrosine oxidation by addition of pyridoxal phosphate. 

The indirect evidence for the transamination is that only four atoms of oxygen are required to oxidize one molecule of tyrosine to acetoacetate, a-ketoglutarate stimulates the oxidation, no free ammonia is produced in the reaction, the oxidation is decreased upon removal of the transaminase coenzyme by ammonium sulfate precipitation or by dialysis, and there is an increase in the rate of tyrosine oxidation upon addition of pyridoxal phosphate. When p-hydroxyphenylpyruvate was used as the substrate addition of keto acids did not increase the rate of oxidation. 

Direct evidence for the transamination step was the demonstration of the formation of glutamate from a-ketoglutarate during tyrosine oxidation and the accumulation of p-hydroxyphenylpyruvate under anaerobic conditions, or in the absence of ascorbic acid ( 07). The crowning step to this Ust of evidence is the isolation of a specific transaminase for tyrosine by Canellakis and Cohen ( 08). 

Tyrosine transaminase was purified about a hundredfold from acetone powder extracts of dog liver (the most potent source of the enzyme) by dialysis and alcohol fractionation. The presence of a phenolic hydroxyl, an alpha amino, and an alpha carboxyl group were required for a compound to be active as a substrate in this transamination system. Only the L-amino acid was attacked and the only effective keto acid is a-ketoglutaric acid. The enzyme, as would be anticipated, required pyridoxal pho hate as a coenz3rme. It contiuns copper but this apparently is unrelated to its... 

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