Thiamine added to a vitamin B_6-free medium caused growth inhibition and respiratory deficiency in Saccharomyces yeasts under aerobic conditions, and concomitant addition of pyridoxine prevented completely the thiamine effects. The respiratory deficiency occurred as the result of a sequence of events as follows: (1) accumulation of thiamine in the free form in the cells, (2) decrease in vitamin B_6 Content, (3) reduction in the activity of δ-aminolevulinate synthase, (4) heme deficiency, (5) cytochrome deficiency, and (6) respiratory defi-ciency. However, the growth inhibition was shown to be partially due to the respiratory deficiency, and the participation of some other events caused by the thiamine-induced vitamin B_6 deficiency was suggested.
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