138 LIGHT AND LIFE 



to a ternary complex. If a semiquinone intermediate is formed, as 

 the spectrum indicates, there should also be an electron spin resonance 

 signal, but none could be observed. A spin resonance absorption 

 band could easily be missed if it were diffuse. Like many flavoproteins, 

 the acyl-CoA dehydrogenase occurs in an organized subcellular par- 

 ticle. The particulate environment obscures details of function, but 

 it is not known to what extent functions are modified or missed when 

 such enzymes are examined in the isolated state. 



Lipoic dehydrogenase. This enzyme, initially characterized by its 

 catalysis of dye reduction by DPNH (24, 20) and termed diaphorase 

 has recently been identified by Massey (15) as lipoic dehydrogenase. 

 It is an FAD enzyme, noteworthy among flavoproteins in that the 

 fluorescence of the flavin is not quenched in the enzyme complex but 

 is enhanced about two-fold over that of free FAD. The absorption 

 bands of the FAD are bleached only about 15 per cent in the presence 

 of excess DPNH, but one mole equivalent of DPNH in a dilute 

 anaerobic system completely quenches the flavin fluorescence, sug- 

 gesting the formation of a pyridine nucleotide complex. Inhibition 

 with arsenite (15a) and the behavior of a 530 m/x, absorption band 

 produced by reduced substrate (15b) are interpreted as evidence for 

 vicinal dithiol groups which interact with the bound flavin to form 

 free radical intermediates. Massey has also postulated the participa- 

 tion of a pyridine nucleotide free radical on the basis of the above 

 observations and an unusual catalytic effect of DPN in the reduction 

 of lipoic acid by DPNH. 



Flavoproteins and molecular oxygen. Most flavoproteins in the 

 reduced form are reoxidized slo^vly by molecular oxygen, with for- 

 mation of hydrogen peroxide. A few special ones, such as xanthine 

 oxidase (1), L-amino acid oxidase (22), and lactate oxidase (28), 

 react rapidly and catalytically with molecular oxygen as the ac- 

 ceptor. The lactate oxidase, crystallized and characterized by Sutton, 

 is of particular interest. Pyruvate and j^eroxide, produced in the 

 initial reaction step, remain bound in some form to the enzyme 

 and react in a second step to form acetate and carbon dioxide. 

 No free pyruvate or peroxide can be detected as intermediates or as 

 products of the reaction. Concentrated solutions or slurries of this 

 enzyme give a weak electron sj)in resonance signal, the intensity of 

 which is dependent upon the concentration of lactate present (6) . 



The microsomal cytochrome reductase. This enzyme was isolated 

 by Strittmatter and myself (27) from calf liver microsome particles. 

 It catalyzes a rapid and specific reduction of the microsomal cyto- 



