The study of interaction of 4-aminobutyrate transaminase with 5'- 6'-methyl derivates of PLP demonstrated that only the former was capable of forming a catalytically active holoenzyme possessing 0.37 activity of the native holoenzyme and a low affinity substrates. This compound interacts with the apoenzyme at a slower rate than does PLP; it has a reduced affinity towards apotransaminase (Km = 1.10(-4) M) and is replaced from the active site by native coenzyme. The other analog of pyridoxal-5'-phosphate forms a catalytically inactive complex with the apoenzyme; the other analog is not replaced from the active center by native coenzyme and non-competitively inhibits the reconstruction of apotransaminase (Ki = 2.10(-5) M).