Transaminase of Branched Chain Amino Acids

It has been established that the first reaction in the metabolism of the majority of amino acids is transamination and many transaminases have been reported. Branched chain amino acids (e.g., valine, leucine, and isoleucine) are very similar in structure and are essential for animal nutrition. It is known that they have a competitive effect on each other in nutrition (1, 2) and that valine is glycogenic, while leucine is ketogenic and isoleucine is both (3—5). In spite of their physiological importance, the mechanism of their transamination in animals is not fully understood. There are several reports showing that they are transaminated (6—9), but for previous studies the enzyme preparations used were crude homogenates of various tissues, and branched chain amino acids were among the other amino acids of which the transamina-tion was studied. Therefore, it is not known whether some transaminase with broad sub-strate specificity acts on branched chain amino acids as well as other amino acids, or whether former have their own specific transaminase and if so, whether this enzyme is common to them all or whether each branched chain amino acid has its own specific transaminase. Studies on transaminases for branched chain amino acids in micro-organisms have suggested that they do not have a specific enzyme, but that their transamination is catalyzed by an enzyme which also trans-aminates a number of other amino acids * Present address: The Institute for Enzyme Research, School of Medicine, Tokushima University, Tokushima. Requests for reprints should be sent to this address. (10—13). This report is on the distribution of transaminase activities of the branched chain amino acids in various tissues of rats and also reports the partial purification and properties of a branched chain amino acid:2-oxoglutarate aminotransferase EC 2.6.1 from hog heart. It is concluded that this enzyme is specific for branched chain amino acids and that these compounds are all transaminated by the same enzyme. Preliminary work on this subject has been reported (14).