Besides conventional methods, transminase activity can also be followed by an isotopically labelled substrate, i.e. by an
amino acid with a15N labelled amino group. The application of the labelled substrate enables their study in vitro as well as in vivo. This method
was applied for following the tryptophan transaminase activity in germinating maize plants. The enzym preparations used for
in vitro analysis were purified on a Sephadex G 100 column. For in vivo experiments the15 N substrate was introduced by vacuum infiltration into the plants leaves. The amino acids originated by transamination are
chromatographically separated and after purifying by paper electrophoresis, the atom excess15N is determined. In maize tissue originated glutamic acid (from infiltrated α-ketoglutarate substrate), which in the course
of further reactions was transformed into aspartic acid and asparagine. The results found so far by studying the low activity
of tryptophan-transaminase in vivo entitle us to improve our method of transaminase activity determination by using15N labelled-substrate.