Ornithine aminotransferase pyridoxal 5 -phosphate

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

L-Canaline is an ineffective antimetabolite of L-ornithine since it has little ability to antagonize ornithine-dependent reactions. On the other hand, it forms a covalently bound Schiff-base complex with the pyridoxal phosphate moiety of Bg-containing enzymes. As such it is a potent inhibitor of many decarboxylases and aminotransferases that utilize this vitamin. 

A question of stability. Pyridoxal phosphate (PLP) is a coenzyme for the enzyme ornithine aminotransferase. The enzyme was purified from cells grovm in PLP-deficient media as well as from cells grown in media that contained pyridoxal phosphate. The stability of the enzyme was then measured by incubating the enzyme at 37°C and assaying for the amount of enzyme activity remaining. The following results were obtained. 

Fig. 38.14. The ornithine aminotransferase reaction. This is a reversible reaction dependent on pyridoxal phosphate, which normally favors ornithine degradation.

Tryptophan administration to rats results in the stabilization of a number of enzymes for which tryptophan is not a substrate. These include tyrosine aminotransferase (Cihak et al., 1973), phosphoenolpy-ruvate carboxykinase (Ballard and Hopgood, 1973), threonine dehydratase (Peraino et al., 1965), and ornithine aminotransferase (Chee and Swick, 1976). With these enzymes, the decreased lability cannot be shown in vitro, so it is doubtful whether tryptophan itself is the active compound. Perhaps a tryptophan metabolite either binds to the enzymes or affects the dissociation of substrates or cofactors. This latter possibility could be important in the case of the aminotransferases, which have pyridoxal phosphate cofactors, since Litwack and Rosen-field (1973) demonstrated a correlation between the degradation rate constants of a number of enzymes and the rate of coenz)nne dissociation from the molecules. 

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