WHY SACCHAROMYCES-CEREVISIAE CAN OXIDIZE BUT NOT DECARBOXYLATE EXTERNAL PYRUVATE

Protoplasts of the yeast Saccharomyces cerevisiae oxidized externally added pyruvate by pyruvate oxidase system but were not able to decarbo xylate it anaerobically by pyruvate decarboxylase at pH 6.4 in isotoni c solutions. The decarboxylation set in hypotonic solutions in which t he integrity of the plasma membrane was being impaired. Yeast cells in cubated with [1-C-14]pyruvate accumulated radioactivity under conditio ns allowing oxidation of pyruvate, but virtually no pyruvate was taken up when the oxidation had been arrested by inhibition or mutation. In view of a large difference between K-m for pyruvate of pyruvate decar boxylase (30 mM) and of pyruvate oxidase (0.16 mM), the results may be accounted for by the assumption that transport of pyruvate across the yeast plasma membrane is trans-inhibited by relatively high concentra tions of intracellular pyruvate. This arrangement would allow utilizat ion of external pyruvate by the cell energy-transforming machinery and , at the same time, prevent it wastage by futile decarboxylation.