Regulation of Pyruvate DecarboxylaseIn VitroandIn Vivo

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Abstract
Results presented in this paper strongly support the view that regulation of the key enzyme of alcoholic fermentation, pyruvate decarboxylase (PDC), is achieved in a number of ways, all associated with possible lowering of the cytoplasmic pH during anoxia. These mechanisms include not only the well-known acid pH optimum of PDC, but also long-term, reversible changes in characteristics of the enzyme established both in vitro and in vivo. Following transfer of desalted extracts from pH 6.0 to 7.4, maximal activity of PDC was decreased, while there was a considerable increase in the lag before maximal activity was reached. Similar changes in enzyme characteristics were observed when wheat (Triticum aestivum L. cv. Gamenya) roots and rice (Oryza sativa L. cv. Calrose) coleoptiles were transferred from anoxic to aerobic solutions, provided PDC was assayed within 10 min of the start of maceration. All of the above changes were usually readily reversible when extracts were returned to pH 6.0, or when plants were returned to anoxic solutions. Additional regulation of PDC would be achieved by the S0.5 for pyruvate which is 0.75 mol m−3 at pH 6.0, 1.0 mol m−3 at pH 6.8, and 2.5 mol m−3 at pH 7.4; the latter is well above estimates for pyruvate concentrations in the cytoplasm of aerated tissues. We assess that the combined effects of the acid pH optimum, the high S0.5 at pH 7.4 and the long-term decreases in activity observed during incubation at pH 7.4 would reduce PDC activity in aerobic cells to at most 7% of the activity in anoxic cells. Possible additional controls for the pathway of alcoholic fermentation are briefly considered.