Intracellular location of enzymes involved in citrate production by Aspergillus niger

The intracellular distribution and maximal activities of nine enzymes involved in the biosynthesis and degradation of citric acid in Aspergillus niger were determined under conditions of growth and of citric acid production. Under these conditions the intracellular location of the enzymes in most cases resembled that described for other filamentous fungi. Pyruvate carboxylase was found predominantly or exclusively in the cytosol. A single isoenzyme of NADP–isocitrate dehydrogenase was present, which appeared to be localised in the mitochondrion. No significant differences in maximal enzyme activities were observed except for NADP–isocitrate dehydrogenase, which showed decreased activity in production-phase mycelia. The results obtained support the scheme proposed by C. P. Kubicek for the intracellular organisation of citric acid formation but provide little evidence that this process is controlled at the level of the biosynthesis of any of the enzymes examined here. Key words: pyruvate carboxylase, citric acid production, enzyme compartmentation, Aspergillus niger.