68 



VITAMINS A AND CAROTENES 



will probably be found to have a large place in the general metabolism of 

 vitamin A. Its properties will therefore be reviewed here in some detail. 

 Two alcohol dehydrogenases are known, which with cozymase as coenzyme 



0.4 - 



03 



02 - 



0.1 - 



<i» 04 



0.3 



0.Z 



0.1 - 



- 



ifar 



■ refinene 2 



300 20 40 60 BO 400 



wavelength'' mjj 



Fig. 16. The action of alcohol dehydrogenase from the frog retina on retinenci and 

 retinene2. Each preparation included retinene dissolved in 1% digitonin solution, 

 0.7 mg. of reduced cozymase per milliliter, 5.5 mg. of nicotinamide per milliliter, and 

 an extract of homogenized frog retinas in il//30 phosphate buffer, pH 6.81. The con- 

 trols differed only in that the retinal extracts were replaced either with the same 

 extract which had boon boiled for i^ minute (upper figure) or with the j)liosphate 

 buffer alone (lower figure). The experimental and control mixtures were incubated 

 for 2 hours at 23°. Methanol was added to each to a concentration of G0%, and they 

 were extracted with hexane. The spectra of the hexane extracts are shown. Those 

 from the controls (solid circles) show the unaltered retinenes; those from the enzyme 

 mixtures (open circles) show complete reduction to the corresponding vitamins A. 

 (From G. Wald.") 



