Abstract
Since dihydrothiamine (DHT) has been assumed as an intermediate or a precursor in the process of thiamine biosynthesis, the existence of DHT-like substances in leaves of some ascorbic acid-rich plants, mainly of Iridaceae, was examined. Samples were homogenized with 60 % ethanol or 2 % solution of metaphosphoric acid under ice-cold condition using a Waring blender, followed by centrifugation to separate the supernatant, in which the thiamine was measured by usual cyanogen bromide method before and after the oxidation treatment with 2,6-dichlorophenolindophenol. The amount of the DHT-like substances was calculated by substracting thiamine contents before the oxidation from that after the oxidation. In only two (Iris tectorum Maxim and I. pseudoacrorus L.) out of eight kinds of plants tested, less than 5〜6μg % of DHT-like substances were found, but in others not at all. DHT was more stable in homogenate than in equeous solution, but ascorbic acid had no ability to improve the stability of DHT in its aqueous solution at pH 4.0 or 7.3,showing inversely a tendency to accelerate the decomposition of DHT.
