Bioregulation of yeast plasma membrane ATPase and carnitin acetyl transferase (CAT) activity at defined metabolic states

Abstract

When glucose sensitive yeasts ( Saccharomyces cerevisiae H 1022) are grown aerobically under glucose limitations in continuous culture, two different metabolic states are obtained. In the first state, which occurs at low dilution rates, the metabolism is purely respirative (derepressed cells) and glucose is exclusively oxidized. The second state is found at dilution rates supercritical for pure respiration (oxide-reductive); a marked increase in RQ-values, accumulation of ethanol, and a drop in biomass are observed (Fiechter et al., 1981, Adv. Microbiol. Physiol. 22, 123–183; Käppeli et al., 1985, J. Gen. Microbiol. 131, 1411–1416). Plasma membrane ATPase and carnitine acetyl transferase (CAT) activities have been measured in asynchronous and stable synchronized chemostat cultures for regulatory studies. The investigation of the plasma membrane ATPase activity showed low activity of around 6 mmol g −1h −1 phosphate released (referred to protein) under respiratory conditions with a relatively broad pH optimum between pH 6 and 7. High specific ATPase activity (> 15 mmol g −1h −1) was found in cells grown at high rates (sharp pH optimum at 6.2). The significant change in the two defined metabolic states might be positively correlated with transport functions of substrates or ions and with maintenance of pH. Consequently, attention was paid to the opposite behaviour of e.g. respiration vs. ethanol formation and plasma membrane ATPase activity. Purely respiring cells excrete no ethanol and plasma membrane ATPase activity is low, whereas in the oxido-reductive state alcohol is formed and plasma membrane ATPase activity is high. CAT activity is extremely low under conditions of glucose repression and high during oxidative metabolism. A unique metabolic property is the spontaneous synchronization of the cells in continuous culture under derepression. Investigation of the plasma membrane ATPase activity during the cell cycle showed an increase of almost 50% between early S-phase and late M-phase. In contrast, activity of CAT increased at the onset of a new cell division cycle in a ratio close to two. Thus, not only extracellular parameters are important for the ATPase regulation but also time setting for sampling within the cell cycle.