IV. lUOCUKMlCAL SYSTEMS 351 



the liver enzyme could be easily deinonstrated, whereas there was no clear 

 evidence of such a mechanism in the case of the milk enzym(\ 



Another dilYerence between liver aiul milk xanthine oxidase was stu(hed 

 by Richert el al.,'^ who showed that antabuse (tetraethylthiuram disulfide) 

 inhibited the rat liver oxidase but did not alTect milk xanthine oxidase. 

 Richert and Westerfeld"'-*' "" and Remy aiul Westerfeld"*' iia\-e made an 

 extensive study of the xanthine oxidase activities of a variety of animal 

 tissues. 



c. Mechanism of Action of Liver Flavoprotein 



The liver aldehydase oxidizes a variety of aldehydes such as acetald(>liyde, 

 propaldehyde, butalilehyde, crot()naldehyd(\ benzaidehytle, salicylalde- 

 hyde, and glycollic aldehydes. Formaldehyde is not oxidized, but this may 

 be due ti) the denaturing; elTect of formaldehyde on the enzyme. The action 

 between aldeliyde ami oxyjj;en is not direct but proceeds as follows: 



Crotonaldehyde + flavoprotein — ^ 



Crotonic acid + reduced ila\'oprotein 



Reduced enzyme + 0> -^ hjizyme + II2O2 



The presence of catalase in tiie enzyme j)rei)aration causes all the perox- 

 ide to be decomposed to water and oxyj^en. 



Oxidation-reduction indicators such as metiiylene blue, nitrate, and cyto- 

 chrome c can be reduced by the enzyme, but the catalytic reductions of 

 nitrate and cj^tochrome c are too slow to be of physiological significance. 



10. D- Amino Acid Oxid.vse 



The existence of this enzyme was in dispute for years before Krebs**'-- ^^ 

 extracted it from the kidney and liver of rats. He clearly distinguished it 

 from L-amino acid oxidase in 193').**' Warburg and Christian'' showed that 

 the prosthetic group of this enzyme is flavin adenine dinucleotide. Xegelein 

 and Bromel**^ isolated the apoenzyme portion in a high degree of purifica- 

 tion and estimated molecular weight at 70,000. There is no absolute agree- 



"D. A. Hichort, H. V:uulorlin<ic. and W. W. W(>s((Mfc!(i, J. liiol. Chcm. 186. 2()1 



(1950). 

 '» D. A. Ricliort. and W. W. Westerfeld, Proc. i^oc. Exptl. Biol. Med. 76, 252 (1951). 

 "• W. W. Westerfeld and D. A. Ricliert, ./. Biol. Cficm. 184, 163 (1950); 192, 35 (1951). 

 «' C. Remy and W. W. Westerfeld, J. Biol. Chem. 193, 059 (1951). 

 « H. A. Krebs, Z. physiol. Chvm. 217, 191 (1933). 

 " H. A. Krebs, Z. phijsiol. Chcm. 218, 157 (1933). 

 " H. A. Krel)s, Biochcm. J. 29, UVM (1935). 

 " E. Negelein and H. Bniniel, Biochcm. Z. 300, 225 (1939). 



