Vitamin A and isoprenoid synthesis in the rat

1. Vitamin A-deficient rats were compared with similar animals given small amounts of vitamin A sufficient for adequate growth and with animals given large amounts of vitamin A. The effects of pair-feeding and feeding ad libitum were compared. 2. Ubiquinone and cholesterol concentrations in liver were measured at various stages of the deficiency, and the uptake of radioactive mevalonate and acetate into isoprenoid compounds was studied. 3. Ubiquinone concentrations in liver increased markedly in deficient rats compared with adequate controls, and heavy vitamin A supplementation had a further effect in depressing ubiquinone concentrations. These effects were unrelated to food intake or to the size of the organs. 4. Radioactive uptake into ubiquinone was often greater in deficient livers, especially during the early stages of the experiments, but the effect was not consistent. 5. Cholesterol concentrations were usually higher in deficient livers and these were more affected by the feeding regimen. 6. No consistent effect of vitamin A deficiency or of vitamin A dosage on the incorporation of mevalonate into cholesterol or squalene was found. 7. No evidence has been found for a specific effect of vitamin A on isoprenoid synthesis at the metabolic level.     

维生素发现的启示

维生素的发现是生物学史上一个重要的里程碑。较为详细地介绍了维生素发现的艰辛历程,以及应吸取的教益和启迪：科学问题对于科学研究具有重要的定向和指导作用;科学的发现是一个知识不断积累、认识不断深化的过程。只有善于学习和鉴别,对已有的结论经过去粗取精、去伪存真,有选择地继承和发展,才能进行创造和创新。找到问题,提出假设,否定假设,再提出假设,否定之否定,是科学研究基本模式。