326 BIOLOGICAL EFFECTS, OF RADIATION 



Pohl (63), by the technique of photoelectric photometry, also detected 

 three absorption bands in cholesterol, which faded upon irradiation. 

 With the knowledge that complete disappearance of the bands corre- 

 sponded to the destruction of only a trivial fraction of the cholesterol, he 

 concluded that the absorbing substance was present only in minute 

 amount. He found that cholesterol purified via the dibromide, or via 

 allocholesterol, and a sitosterol purified via the dibromide, did not show 

 the usual absorption. 



These simultaneously announced studies left no doubt that an 

 impurity present in ordinary cholesterol and phytosterol is responsible 

 for at least the greater part of the antirachitic activity conferred upon 

 these sterols by irradiation. 



After an interchange of suggestions between the investigators, the 

 impurity, or provitamin, was presently identified as ergosterol. Pohl 

 (64) found that the three known absorption bands of ordinary cholesterol 

 are also exhibited by ergosterol. Moreover, ergosterol showed them in 

 vastly greater intensity than cholesterol. Chemical studies and a 

 biological assay were announced by Windaus and Hess (96). In keeping 

 with expectations, the antirachitic potency of irradiated ergosterol was 

 found to be far greater than that of irradiated cholesterol. At the same 

 time, Rosenheim and Webster (70, 72) reported essentially the same 

 findings, from which they concluded that the precursor of vitamin D is 

 ergosterol, or a sterol of similar constitution. 



Bills, Honeywell, and MacNair (14) confirmed these remarkable 

 discoveries in a w^ay w^hich seemed to leave no reasonable doubt that 

 ergosterol is the contaminant provitamin. By the use of a hydrogen 

 discharge tube, which emits an exceptionally continuous ultra-violet 

 spectrum, they discovered that ordinary cholesterol exhibits a fourth 

 absorption band, X 2600 A. Exactly the same band was shown by 

 ergosterol, making the series XX 2935, 2820, 2700, and 2600 A, with four 

 points of identity instead of three. An indication that the contaminant 

 was ergosterol, and not some sterol with the same absorption bands, was 

 obtained by comparing the rates of destruction. Acetone solutions of 

 ordinary cholesterol, and of ergosterol-free cholesterol plus added ergos- 

 terol, were boiled with potassium permanganate. The absorption 

 bands, identical in intensity at the outset, faded at essentially the same 



rates. 



Despite the convincing evidence that ergosterol is the provitamin in 

 cholesterol, Waddell (84) has recently published experiments which 

 indicate that such is not the case. He found that irradiated cholesterol 

 was many times more effective on chickens, rat unit for rat unit, than 

 ergosterol which had been irradiated either by itself or in the presence of 

 cholesterol. Waddell's experiments contain no obvious flaw, and if 

 confirmed, will necessitate revision of the provitamin concept. 



