IV. BIOCHEMICAL SYSTEMS 15 



IV. Biochemical Systems 



WENDELL H. GRIFFITH and JOSEPH F. NYC 



A. ENZYMES AND COENZYMES 



Studies on choline and its derivatives have emphasized the biochemical 

 importance of these compounds as structural components of tissues, as 

 intermediates in vital metabolic reactions, and as specific chemical reactants 

 of marked biological potency. On the other hand, evidence for the partici- 

 pation of choline or of its derivatives in a specific manner as cofactors in 

 enzymatic systems is meager, although a few reports have linked it or its 

 phosphoric acid ester with phosphatases. Caution is needed in questioning 

 the importance of choline as a component of coenzymes, because relatively 

 little definite information is at hand regarding the functions and properties 

 of the lipoproteins that contain choline phospholipids. 



Kielley and Myerhof^ believe that a magnesium-activated adenosine- 

 triphosphatase (ATPase) of muscle may consist of a lipoprotein with a 

 choline-containing phospholipid as a constituent. The compound was de- 

 void of myosin and actomyosin, and there was no indication that it was 

 another form of myosin ATPase. Its pH optimum was 6.8, and it was 

 strongly inhibited by calcium. Inactivation of the enzyme and hydrolysis 

 of the phospholipid portion by lecithinase of Clostridium welchii paralleled 

 each other. The occurrence of the pyrophosphoric acid ester of choline in 

 the prosthetic groups of acid and alkaline phosphatases has been reported. ^ 

 Other workers* have noted that this ester contains a labile phosphate group, 

 hydrolyzable by crude and not by purified muscle pyrophosphatase, but 

 they are not of the opinion that it is a coenzyme of a phosphatase. Alkyl 

 nitrogen-substituted derivatives of aminoethanol and of choline activate 

 alkaline phosphomonoesterases.* Activation of an ATPase system in rat 

 submaxillary gland by acetylcholine in vitro has been reported.^ 



B. CHOLINE ACETYLASE AND ACETYLCHOLINESTERASE 



Coenzyme A (CoA) appears to be a common coenzyme for most, if not 

 all, of the acetyl-transferring systems, including the acetylation of choline. 

 The constituents of one form, at least, of this important thermostable com- 

 pound include adenosine-2'(or 3')-phosphate,^' ^ pyrophosphate,^ panto- 



1 W. W. Kielley and O. Myerhof, J. Biol. Chem. 176, 591 (1948); 183, 391 (1950). 

 ^ W. Kutscher and H. Sieg, Naturwissenschaften 37, 451 (1950). 

 3 J. Roche, N.-V. Thoai, and N.-V. Thiem, Compt. rend. soc. biol. 145, 168 (1951). 

 ^ R. Granger and J. Fraux, Ti-av. soc. pharm. Montpellier 5, 48 (1945-1946); 6, 93 



93 (1946-1947). 

 5 K. P. DuBois and V. R. Potter, /. Biol. Chem. 148, 451 (1943). 

 « F. Lipmann, N. O. Kaplan, G. D. Novelli, and L. C. Tuttle, J. Biol. Chem. 186, 



235 (1950). 



