Humans require about half of the amino acids in their diet for growth and maintenance of normal nitrogen balance. Essential amino acids such as Arginine and Histidine are provided in the diet to meet an individual’s metabolic needs whereas the non-essential amino acids like Alanine, Glutamate and Aspartate are not obtained in the diet but are synthesized in adequate amounts within the body (Marshall & Bangert, 2005). Some of these amino acids are synthesized by a one- step straight forward reactions because they have simple structures, for instance, Glutamate can by synthesized by adding ammonia to a-ketoglutarate and when another ammonia molecule is added to that glutamate, Glutamine is formed. The rest of the simple reactions involve the transfer of amino group (transamination) from glutamate or glutamine to a central metabolite to make the required amino acid e.g. Aspartate is synthesized by the transfer of an ammonia group from glutamate to oxaloacetate.
Fig 1: Transamination process
Transamination therefore is defined as the process by which an amino group, usually from glutamate, is transferred to an -keto acid, with formation of the corresponding amino acid plus -ketoglutarate (Devlin, 2011). An amino acid contains an amine (NH2) group while a keto acid contains a keto (=O) group. What happens in transamination is that the NH2 group on one molecule is exchanged with the =O group on the other molecule. The amino acid becomes a keto acid, while the keto acid becomes an amino acid (fig 1). The presence of the -amino group keeps amino acids from oxidative breakdown, however removing the -amino group is necessary for producing energy from any amino acid. This mechanism provides a route for the re-distribution and subsequent excretion of excess nitrogen as urea because the accumulation of nitrogen in the form of ammonia is toxic, these reactions are catalysed by transaminases (aminotransferases). These enzymes are found in the cytosol of cells throughout the body – especially those of the liver and muscles. (Harvey, et al 2005) The two important enzymes used in the clinical diagnosis of human disease are Aspartate Aminotransferase (AST) and Alanine Aminotransferase. Aspartate aminotransferase (ALT).
ALT, catalyzes the reversible transfer of the amino group of alanine to -ketoglutarate, resulting in the formation of pyruvate and glutamate whereas AST transfers amino groups from glutamate to oxaloacetate, forming aspartate, which is used as a source of nitrogen in the urea cycle (Harvey, 2005). Transamination is used not only for amino acid synthesis, but also for the degradation of excess amino acids. When blood sugar is low, the body metabolizes proteins to amino acids at the expense of muscle tissue. The preference of liver transaminases for oxaloacetate or alpha-ketoglutarate plays a key role in funnelling the nitrogen from amino acid metabolism to aspartate and glutamate for conversion to urea for the excretion of nitrogen. In similar manner, in muscles the use of pyruvate for transamination gives alanine, which is carried by the bloodstream to the liver. This overall reaction is termed the glucose-alanine cycle.
Fig 2; Diagram showing the role of transaminase in balancing the available amino acids for metabolism and excretion in the human body.
The transaminase enzymes are important in the production of various amino acids, and by measuring the concentrations of various transaminases in the blood, clinical diagnosis can be made. This write up will be used to determine the activity of Aspartate Amino transferase (AST) for two patients. Results obtained will be compared to the accepted reference range for Brighton laboratory (0-50 U/L) that indicates what level of concentration is normal. This write up also discusses the findings of the results in the context of the clinical