PROPERTIES AND CATALYTIC ACTIVITIES OF OMEGA-HYDROXY DERIVATIVES OF VITAMIN B6

This paper treats of the catalytic activities of ω-hydroxy derivatives of vitamin B6. ω-Hydroxypyridoxamine, obtained by treatment of pyridoxamine N-oxide in acetic anhydride, was phosphorylated to ω-hydroxypyridoxamine 5′-phosphate using P2O5 and H3PO4. Preparation of ω-hydroxypyridoxal 5′-phosphate was performed by the transamination reaction between ω-hydroxypyridoxamine 5′-phosphate and pyridine-2-aldehyde. The nonenzymatic transamination reaction between ω-hydroxypridoxamine and α-ketoglutarate proceeded a little faster than the reaction between pyridoxamine and the acid. ω-Hydroxypyridoxal 5′-phosphate was found to be superior to pyridoxal 5′-phosphate as the coenzyme for the mammalian glutamic-oxaloacetic transaminase as judged from the maximal velocity (Vmax) of the reaction catalyzed by the reconstituted holoenzyme. On the other hand, the affinity of the analogue for E. coli tryptophanase and the Vmax value of the decomposition of tryptophan dependent on the analogue-bound enzyme were significantly smaller than those in the case of the normal coenzyme. These facts suggest that the groups at the active site of GOT are flexible enough not to interfere with the modification of the hydrophobic 2-position of the coenzyme. On the contrary, the 2-methyl group seems to play an important spatial role in interaction with the coenzyme-binding site of apotryptophanase such that the alteration in this group would lead to variations in the catalytic site of the resulting holoenzyme. © 1969.