Solutions of about equal amounts of 7dehydrocholesterol-3-C14 and (inactive) lumisterol, were irradiated with ultra-violet light (265-280 mp) for short periods of time, resulting in approximately 10% conversion of the provitamin. Similar experiments were performed with labelled ergosterol (obtained by growing yeast in a medium containing acetate-l-C"), admixed with inactive lumisterol, and tachysterol,. Isolation of the compounds present in the irradiated solutions was effected by precipitating the provitamin D with digitonin and separating the remaining components according to their different rates of reaction with maleic anhydride (tachysterol > vitamin D > lumisterol). The specific radio-activities of the products isolated gave clear-cut evidence that lumisterol does not play any part as an intermediate in the main route of the reaction provitamin D + tachysterol nor in the conversion provitamin D --+ vitamin D.Besides, lumisterol seems to be formed at a relatively low rate under the conditions employed, which are comparable with those generally used in vitamin D synthesis. As to tachysterol, here conclusions were somewhat less definite because this compound has rather a high rate of photochemical formation and decomposition and is difficult to purify. The specific activity measurements, however, strongly supported the conclusion that also tachysterol is not a necessary intermediate in the vitamin D formation. Thus the results are contradictory to the traditional scheme provitamin D + lumisterol+ tachysterol --+ previtamin D e v i t a m i n D, and are strongly in favour of a reaction pattern of the following type: hv hv hv 1) Other papers of this series: E. Hauinga, J. P. L. Studies on vitamin D a d refated compounds ZV. 74 (1955) RECUEIL 1231 vi min D
