Historic background Braunstein-Snell hypothesis

The original discovery leading to our present interest in pyridoxal-P-dependent biological reactions was made in 1934 by Paul Gyorgy [1] in the United States (Western Reserve University, Cleveland). He identified a nutritional factor that could cure a specific dermatitis produced in young rats when they were reared on a deficient diet. Gyorgy called this factor vitamin which was subsequently shown to be pyridoxine (Fig. 1,1). The term vitamin is currently used to include a number of closely related substances of dietary origin (Fig. 1,1,2 and 3), which, in animals, are metabolised to produce the enzymically active form, pyridoxal phosphate (Fig. 1, 5). The pathways for these transformations [2] are shown in Fig. 1. 

Pyridoxal phosphate and its close relative, pyridoxamine phosphate (Fig. 1, 6) participate in a multiplicity of different biological processes, most noteworthy amongst these being the reaction involved in the metabolism of amino acids [3-6]. Several dozen enzymes are known whose activities are dependent on the presence of pyridoxal phosphate in their active sites. Our current view regarding the niechanisms of pyridoxal-P-dependent transformations is based on the inspired suggestion originally made in 1953, independently by Braunstein and Shemyakin [7] in the Soviet Union, and Snell and his coworkers [8] in the United States. Before considering the basic tenets of the Braunstein-Snell hypothesis we examine the nature of the chemical problems involved in the metabolism of amino acids. In the later sections we shall note that the key event in the metabolism of amino acids is either the 

Michael I. Page (Ed.), The Chemistry of Enzyme Action 1984 Elsevier Science Publishers B.V. 

In the light of this background we consider an enormous body of experimental work on pyridoxal-P-dependent enzymic reactions which has accumulated since the enunciation of the Braunstein-Snell hypothesis. Broadly speaking, at a crucial stage in these reactions, the cleavage of either the C -COOH or C -H bond is involved. In Sections 2-5 transformations belonging to these two classes are discussed sequentially with major emphasis on the description of events which occur on the 

For the arrangement of substituents around the pyridinium ring, see comments on p. 367. 

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