SIDNEY 1'. VELICK 



139 



chrome-bj by DPNH. The red shilt ot the flavin absorption l)ands 

 (Fig. 21) is typical of flavojirotcins, but the enhancement of the 

 shoulder around 485 ni^u is unusually pronoiaiced. The FAD on 

 this enzyme, of 40,000 g molecular Aveight per mole, is strongly 

 bleached by DPNH, and a pyridine nucleotide complex is formed 



Oxidized 



Excess 



DPNHorNa2S20 



320 300 350 400 450 

 WAVE LENGTH (m|j) 



Fig. 21. The absorption spectra of the bound flavin in the microsomalcytochrome-bs 

 reductase; soHd Hne, oxidized; dashed line, reduced (27). 



(25). The complex in this case is recognized by an absorption band 

 at 315 vajj., most clearly seen in difference spectra (Fig. 22) . The 

 pyridine nucleotide flavoprotein complex is only formed by admix- 

 ture of cross conjugate pairs, DPNH and oxidized flavoprotein or 

 DPN and reduced flavoprotein. It dissociates when the complex 

 is reoxidized by a variety of reagents, as it must do also in the rapid 

 catalytic reaction with cytochrome-b-. The reductase contains a sulf- 

 hydryl group that is essential for the binding and oxidation of DPNH 

 (27) . Two additional sulfhydryl groups can be distinguished from 

 this one by their chemical reactivity and do not seem to be directly 

 involved in the catalysis (26) . In the pyridine nucleotide enzyme 

 complex the flavin is fully bleached and hence appears to be in the 

 fully reduced form. If this is true, the pyridine nucleotide is in the 

 oxidized state, and the 315 m/i, absorption band corresponds to the 

 interactions that occur Avith DPN and inhibitor on LDH and other 

 dehydrogenases. Such a band could arise, for example, from inter- 



