Carboxylase Levels and Carbon Dioxide Fixation in Baker's Yeast
AUTOR(ES)
Cazzulo, J. J.
RESUMO
Levels of pyruvate carboxylase (PC), phosphopyruvate carboxylase (PEPC), and malate dehydrogenase (decarboxylating) were compared in wild-type bakers' yeast (I), a cytoplasmic-respiratory mutant (II), a biotin-deficient wild-type yeast (III), and a biotin-deficient respiratory mutant (IV). PC activities were greatly reduced in III and IV, whereas PEPC was reduced in II and IV. Malate dehydrogenase (decarboxylating) could not be detected in any of the yeasts. With yeast I growing on glucose as the sole carbon source, PEPC decreased to negligible levels during the logarithmic phase of growth (glucose repression effect), whereas PC increased. Both enzymes reverted to their original levels during the stationary phase, when glucose in the medium was exhausted. In agreement with the leading role of PC for CO2 assimilation, the rates of 14CO2 fixation in yeasts I and II were approximately equal and were much higher than that in yeast IV. With I and II, most of the 14C was distributed similarly in oxalacetate derivatives; with yeast IV, most of 14C appeared in a compound apparently unrelated to CO2 fixation via C4-dicarboxylic acids.
