Biochemistry of Amino Acids - A. Meister 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Oxidative Deamination
Introduction

The Deamination of Amino acids leading to The formation of the corresponding α-keto acids was first discovered by Neubauer [110] and Knoop [111]. It proceeds via two primary general mechanisms—Transamination (p. 208) and Oxidative Deamination. Oxidative deamination of Amino Acids was first thoroughly investigated by Krebs [112–114], who established that Liver and Kidney preparations catalyze The oxidation of both D- and L-isomers of amino acids.

The enzyme catalyzing the oxidation of D-amino acids was successfully separated from the enzyme system responsible for the oxidation of L-amino acids. A number of subsequent studies confirmed the presence of D-Amino Acid Oxidase in the Kidneys and liver. The oxidative deamination of L-amino acids reported by Krebs is likely explained by the combined action of transaminases and Glutamate dehydrogenase (p. 234). Nevertheless, several preparations of L-amino acid oxidases have been obtained, and the existence of such Enzymes is hardly open to doubt.

The oxidation reaction of L- and D-amino acid isomers by their respective oxidases can be expressed as follows:

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The general D- and L-amino acid oxidases studied to date have proven to be Flavoproteins [115–119]. During the action of these oxidases, two hydrogen atoms are removed from The amino acid molecule, and hydrogen peroxide is formed, as shown below:

The Formation of the imino acid enclosed in square brackets in the equation above has not yet been experimentally proven. It is generally accepted that the Hydrolysis of this hypothetical imino acid occurs spontaneously. In the absence of catalase during the enzymatic oxidation of an amino acid, the formation of a homologous lower carboxylic acid and Carbon dioxide is observed:

As is known, hydrogen peroxide reacts with α-keto acids in the following non-enzymatic reaction:

In the presence of catalase, hydrogen peroxide is degraded; in this case, the oxidative deamination reaction of the amino acid proceeds According to the overall equation:

If catalase is present in the system, the hydrogen peroxide generated during the oxidation of Amino acids can also oxidize ethyl alcohol [120]:

This reaction serves as a classic example of the diverse peroxidase-like Reactions Catalyzed by catalase.