IV. BIOCHEMICAL SYSTEMS 507 



TPN may be readily converted to DPN by phosphatases from a number 

 of sources,"' ^^^ by yeast,''^ and by "cyclophorase" preparations from rabbit 

 kidney and rabbit hver.^^" 



K. DESTRUCTION OF DPN AND TPN 



Some of the ways in which these coenzymes can be degraded have al- 

 ready been described, since most of the reactions described under biosyn- 

 thesis (p. 506) are reversible. Actually, it has long been known that many 

 animal tissues, plants, and microorganisms can enzymatically split these 

 coenzymes. 1^1 Ohlmeyer'^- and HeiwinkeP^^ identified adenyhc acid as a 

 breakdown product of DPN. Das and von Euler'^^' '" found inorganic 

 orthophosphate when DPN was destroyed in animal tissues. Handler and 

 lOein^*^ identified nicotinamide as a breakdown product when DPN was 

 destroyed by brain, liver, kidney, and muscle preparations from several 

 animal species. Mann and QuasteP^^ also called attention to the potent 

 destructive power of certain animal tissues, especially brain, and showed 

 that this rapid breakdown could be inhibited with rather high concentra- 

 tions of nicotinamide. Mcllwain and associates'^^- ^^^ have also studied the 

 destruction of DPN in brain preparations. Govier and Jetter'^" have shown 

 that a-tocopherol phosphate has an inhibitory effect on DPN breakdown 

 by heart DPNase. From these and other studies it appears that the com- 

 mon DPNase of animal tissue splits DPN at the glycosidic linkage between 

 nicotinic acid and ribose. Nason et al."^ have concentrated a DPNase from 

 Neurospora which acts in a fashion similar to animal DPNase. This enzyme 

 is not inhibited by excess nicotinamide, however. Animal tissues also con- 

 tain another type of DPNase which spHts DPN at the pyrophosphate 

 linkage.^" 



These DPN-destroying enzymes are particularly active when the cells 



159 D. R. Sanadi, J. J. Betheil, and B. J. Katchman, Abstr. Am. Chem. Soc. 118th Meet- 

 ing, p. 52C, 1950. 



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!«' F. Schlenk, Advances in Enzymol. 5, 207 (1945). 



i«2 P. Ohlmeyer, Biochem. Z. 287, 212 (19.36). 



163 H. Heiwinkel, Arkiv Kemi, Mineral. Gcol. 13A, No. 19 (1939). 



'«< N. B. Das and H. von Euler, Nature 141, 604 (1938). 



i«6 N. B. Das, Arkiv Kemi, Mineral. Gcol. ISA, No. 7 (1939). 



i«6 P. Handler and J. R. Klein, J. Biol. Chem. 143, 49 (1942). 



16' P. J. G. Mann and J. II. Quastel, Biochem. J. 35, 502 (1941). 



•68 II. Mcllwain and D. E. Hughes, Biochem. J. 43, 60 (1948). 



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