Il6 THOMAS P. SINGER 



TABLE II 



Comparison of DPNH Dehydrogenase and Diaphorase 



DPNH Dehydrogenase Diaphorase 



Reaction „ Ratio Ratio 



, opntS^/ ■ / 1^ i^ma. Fe (CN)e F^,, Fe (CN)6 



(/xM DPNH /mm. /ml.) — ^p — — — — rj- 



t^max Lipoamide V ^^^vpodxnxas. 



DPNH + Fe (CN)6 + + + 150 56 — 



DPNH + Lipoamide 27 — ■ 01 



Conditions of assay: as per IVIassey (pH 6-5) [6]. K,„ for lipoamide — 2 mM 

 for DPNH dehydrogenase, 5 niM for diaphorase. 



activity with cytochrome c may represent a trace contamination with 

 Mahler's reductase, rather than a property of DPNH dehydrogenase, is 

 suggested by the fact that the reactivity with cytochrome c was inhibited 

 by the same substances at the same concentrations as reported for DPNH 

 cytochrome reductase [25, 26] (Table IV). Such trace contamination would 

 not be surprising in view of the fact that these experiments were carried 

 out with the initial soluble extract prior to fractionation. 



TABLE III 



Cytochrome c Reductase Activity of Partially Purified DPNH 

 Dehydrogenase 



Conditions of assay: as per Alahler and Elowe [25]. 



Comparison of the properties of the enzyme described with those of 

 the preparation of King and Howard [7] would be interesting but is 

 rendered difficult by the fact that their detailed data, particularly the 

 assay conditions employed, have not yet been published. In view of the 

 similarities in the extraction procedure, it seems very likely that the 

 enzyme here described is one of the DPNH oxidizing activities detected 

 by these workers on chromatographing their extracts on DEAE cellulose 

 [27]. The presence of several components capable of oxidizing DPNH 

 in their preparation is not surprising in view of the heterogeneous con- 

 stitution of the starting material employed (Keilin-Hartree preparation), 

 which, in turn, might make the deiinite identification of the individual 



