Pyridoxal Phosphate Vitamin B6 as Coenzyme for Transamination

The association between vitamin B6 deficiency and transamination emerged from 1945 when Schlenk and Fisher noted that pyridoxine-deficient rats had a diminished capacity for transamination. In the same year Gunsalus and his colleagues found transamination in Streptococcus faecalis depended on pydridoxal phosphate. The properties of the heat-stable component in purified glutamic-oxaloacetate transaminase were similar to those of pydridoxal phosphate. Later pyri-doxal phosphate was established as an essential coenzyme in many amino acid transformations. 

Back in 1937, Braunstein and Kritzman and, independently, Herbst, had proposed transamination might proceed via a Schiff s base formation. The essential lability of the H atom on the a-C atom was shown with deuterium labelling (1942). a-2H alanine released 2H into the medium during transamination. The label did not appear in glutamate, the end-product. 

Identification of pyridoxal phosphate as coenzyme suggested the aldehyde group on pyridoxine might form an intermediate Schiff s base with the donor amino acid. Pyridoxamine phosphate thus formed would in turn donate its NH2 group to the accepting a-ketonic acid, a scheme proposed by Schlenk and Fisher. 15N-labeling experiments and, later, the detection of the Schiff s base by its absorption in UV, confirmed the overall mechanism. Free pyridoxamine phosphate however does not participate in the reaction as originally proposed. Pyridoxal phosphate is invariably the coenzyme form of pyridoxine. 

A few years later, in 1953, the versatility of pyridoxal phosphate was illustrated by Snell and his collaborators who found many of the enzyme reactions in which pyridoxal phosphate is a coenzyme could be catalyzed non-enzymically if the substrates were gently heated with pyridoxal phosphate (or free pydridoxal) in the presence of di- or tri-valent metal ions, including Cu2+, Fe3+, and Al3+. Most transaminases however are not metal proteins and a rather different complex is formed in the presence of the apoprotein. 

Baldwin, E. (1949.) An Introduction to Comparative Biochemistry. Cambridge University Press. 

---