COENZYMES DERIVED FROM B VITAMINS 125 



contributions which have no direct bearing on the functions of the coen- 

 zymes nor will enzymatic reactions be cited whose existence is not well 

 substantiated. 



Coenzymes Required for the Synthesis and Cleavage of Ester, Acetal, and 

 Amide Linkages 



Most of the high molecular weight compounds — fats, polysaccharides, 

 proteins, nucleic acids, etc. — must be cleaved into simpler substances 

 before they can be absorbed and incorporated into essential structures 

 of cells. This extracellular digestion is catalyzed by a group of enzymes 

 which directly hydrolyze the ester, acetal, and amide linkages of the 

 macromolecules. In intracellular syntheses, their hydrolytic products — 

 simple sugars, fatty acids, amino acids, etc. — are recombined by reactions 

 which recreate the acetal, ester, and amide bonds. These synthetic reac- 

 tions within cells cannot be mediated by the same enzymes which cata- 

 lyzed the hydrolysis, since the intracellular concentrations of amino acids, 

 free fatty acids-, and monosaccharides are probably never high enough to 

 reverse the direction of the corresponding hydrolytic reactions. The for- 

 mation of these larger molecules must proceed by indirect routes by which 

 are incorporated reactions which introduce the energy necessary for the 

 coupling of the component units (usually by the formation of phos- 

 phorylated intermediates) . 



With one exception, there is no evidence that the B vitamins function 

 in either the direct hydrolytic reactions or in the synthetic mechanisms 

 utilizing phosphorylated intermediates. However, each of the B vitamins 

 is required for the production of some of the ultimate units from which 

 the fats, carbohydrates, and proteins are formed, and most of the vitamins 

 are essential for the energy-producing processes which supply the energy 

 needed for the synthetic reactions. 



Inositol as a Coenzyme. One reaction which is probably an excep- 

 tion to the statement in the previous paragraph is the hydrolysis of 

 amylose by a pancreatic enzyme. Inositol appears to function as an 

 essential component of this enzyme, a-amylase. The coenzymatic activity 

 of inositol was first suggested when a highly purified preparation of 

 a-amylase was shown to contain 4.1 mg of inositol per gram. 1 Subse- 

 quently this enzyme was shown to dissociate upon dialysis into a dialyz- 

 able thermostable component (a coenzyme) and a protein having no 

 enzymatic activity. 2 The dissociation of the enzyme during dialysis pro- 

 duces a change in the protein component, so that it is no longer enzymati- 

 cally active even when recombined with the dialyzable fraction. This 

 change, however, does not affect the capacity of the protein for combining 

 with the coenzyme. This was demonstrated by showing that the dialyzed 



