Metabolic Pathways Common to All Amino Acids

In the preceding sections, attention was focused on amino acid metabolism in the intact animal. We now examine the metabolic pathways of individual amino acids, which take place in the cells of various human tissues. The first reaction in the metabolic pathways of many amino acids is the loss of nitrogen through transamination or deamination. Conversely, the biosynthesis of many non-essential amino acids involves the addition of nitrogen to amino acid precursors amination and transamination. Decarboxylation, or loss of C02, is another reaction shared by many amino acids. 

Transamination, often also referred to as aminotransfer, is applied to those enzymatic reactions in which an amino group is exchanged between an amino acid and an a-keto acid. This type of reaction is catalyzed by a group of transferases called transaminases or aminotransferases. They are active in both the cytosol and the mitochondria of most cells. An essential prosthetic group of such enzymes is pyridoxal phosphate, and the reaction is generally of the ping-pong type. 

There are numerous transminases, each specific to a given substrate pair. Some may be primarily mitochondrial others, cytosolic. For example, glutamate-oxaloacetate transminase (GOT), also called aspartate aminotransferase (AST), is primarily a mitochondrial enzyme. AST is extensively used in the diagnosis of heart and liver disorders (see Chapter 5). The AST reaction is represented by Equation (20.7). 

The most common acceptors of amino groups in transamination reactions are a-ketoglutarate, oxaloacetate, and pyruvate. Transamination reactions are readily reversible. Some amino acids, such as lysine, do not transaminate in the manner indicated in Equation (20.7). 

The reason certain amino acids are essential in human diets is because the carbon skeletons cannot be synthesized. It was reasoned formerly that the re- 

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