34 CHOLINE 



Bernheim and Bernheim^*^ prepared a choline oxidase extract of rat liver 

 and showed that it was distinct from the enzyme which oxidized ethyl 

 alcohol. The oxygen uptake and the properties of the end products indi- 

 cated that at pH 6.7 choline was oxidized to betaine aldehyde with the up- 

 take of one atom of oxygen. At this pH betaine aldehyde was oxidized fur- 

 ther at a very slow rate, whereas at pH 7.8 the aldehyde was oxidized 

 rapidly to betaine, presumably by another enzyme system. The above 

 workers found the oxidase system in the liver and kidney of the rat but 

 failed to find it in blood, brain, or muscle. 



Klein and Handler^*^ were able to show that the oxidation of betaine 

 aldehyde required diphosphopyridine nucleotide, but that of choline did 

 not. In the presence of the nucleotide, betaine aldehyde was also oxidized 

 by preparations of rat kidney, brain, and muscle, the activities of these 

 tissues compared to liver being 0.33, 0.13, and 0.10, respectively. 



Mann and his coworkers^^* observed that arsenocholine, like choline, is 

 oxidized to the corresponding aldehyde by the choline oxidase of rat liver. 

 The substitution of nitrogen by arsenic reduces the affinity of the enzyme 

 for the substrate. According to these authors the choline oxidase system 

 consists partly of a dehydrogenase. This was shown by the fact that rat 

 liver extracts, in the presence of choline or arsenocholine, rapidly reduce 

 sodium ferricyanide under anaerobic conditions. It was also observed that 

 choline oxidase in the presence of choline or arsenocholine reduces cyto- 

 chrome c at room temperature. 



An apparent phosphate requirement for choline oxidase activity was ob- 

 served by Angel and Miller. ^^^ When rat liver homogenates were prepared 

 with phosphate, veronal, and citrate buffers at pH 7.8, the choline oxidase 

 activities of the suspensions in citrate and veronal buffers were 18 % and 

 50 %, respectively, of that in the phosphate buffer. Addition of phosphate 

 to citrate and veronal suspensions stimulated the oxidase activity by 25 

 to 50 %, respectively. The phosphate continued to stimulate the choline 

 oxidase activity in the presence of dinitrophenol (10"' M). 



The requirements of the rat liver choline oxidase system in whole liver 

 homogenates have been studied by Williams et al.,^^'^ who investigated the 

 effect of pH on the oxidation of choline by homogenates of rat liver. Only 

 small differences were observed in the activities at pH 6.8, 7.3, and 7.8. 

 The oxidation of choline at pH 6.8 and 7.8 gave sigmoidal oxygen consump- 

 tion curves if allowed to run for 2 hours, while the pH 7.3 homogenate gave 

 maximum activity during the first 10-minute period, with a gradual decrease 



i« J. R. Klein and P. Handler, J. Biol. Chem. 144, 537 (1942). 



1" P. J. G. Mann, H. E. Woodward, and J. H. Quastel, Biochem. J. 32, 1024 (1938). 



1" C. Angel and O. N. Miller, Federation Proc. 11, 435 (1952). 



160 J. N. Williams, Jr., G. Litwack, and C. A. Elvehjem, J. Biol. Chem. 192, 73 (1951). 



