Atmospheric oxygen level influences alcohol dehydrogenase and pyruvate decarboxylase activities in sweetpotato roots

Alcoholic fermentation enzyme activities in sweetpotato (Ipomoea batatas L. Lam, cv. Beauregard) roots varied with atmospheric O2 content (0, 1, 1.5, 5, and 21 % O2) at 22 ± 2 °C. Pyruvate decarboxylase (PDC) activity and alcohol dehydrogenase (ADH) activity were assayed at their experimentally determined pH optima and increased significantly when roots were placed under anoxic atmospheres (generally ≤1.5 % O2). PDC activity increased more than ADH activity in response to anoxia. PDC activity was 21- to 28-fold less than ADH activity under aerobic conditions, but was 6- to 8-fold less under anoxic atmospheres. Both PDC and ADH activities significantly decreased after transfer from anoxic atmospheres to air. However, PDC activity decreased more rapidly and reached a level similar to aerobic conditions, indicating PDC was more sensitive to oxygen availability than ADH. Generally, PDC and ADH activities continued to increase during the first 4.5 d of low oxygen exposure. PDC activity was strongly correlated to ADH activity during the entire 14.5 d exposure period to low oxygen atmospheres. A strong correlation existed between PDC activity and acetaldehyde concentration (R2 = 0.95), but a weak correlation existed between ADH activity and ethanol concentration (R2 = 0.52). Our results explicitly suggest that PDC is likely to be the rate-limiting enzyme in alcoholic fermentation of sweetpotatoes under low oxygen atmospheres.