13C NMR studies of carbon metabolism in the hyphal fungus Aspergillus nidulans.

Abstract

Natural-abundance high-resolution 13C NMR spectra (linewidth, 10 Hz) of the hyphal fungus Aspergillus nidulans have been obtained after growth on glycolytic or gluconeogenic carbon sources. Various polyols, some tricarboxylic acid-cycle intermediates and amino acids, and some phospholipids and fatty acyl compounds are present. The polyols found are mannitol, arabitol, erythritol, and glycerol. The nature of the carbon source has a pronounced effect on the pool sizes of the various polyols. All are present when the fungus is grown on sucrose or sucrose/acetate under strongly aerobic conditions. When grown on acetate, both arabitol and glycerol levels are low, whereas on glycerol erythritol is also hardly detectable. The effect of oxygen is most outspoken in glycolytically grown cultures. Limited oxygenation leads to low levels of arabitol and glycerol. Strong oxygenation changes the ratio of erythritol to mannitol, favoring the C4 polyol. An increase in oxygen supply leads to (i) stimulation of the fluxes through the pentose phosphate pathway and glycolysis, (ii) an overflow of reduced metabolites both at the pentose phosphate pathway level (erythritol and arabitol) and at the C3 level of the glycolytic pathway (glycerol), and (iii) the usual accumulation of mannitol. Upon starvation, glycerol, the other three polyols, and the tricarboxylic acid-cycle intermediates and their associated amino acids disappear in this order. As in yeast, gluconeogenic substrates lead to the synthesis of trehalose, which also occurs when mycelium is grown on acetate/sucrose under limiting aeration. A transient formation of trehalose has been observed upon incubation of starved mycelium, cultured on different substrates, with [13C]glucose.