Reguladtion of Citrate Efflux from Mitochondria Oleaginou and Non‐Oleaginous Yeasts by Adenine Nucleotides

Abstract

The regulation of mitochondrial citrate metabolism has been investigated in oleaginous and non‐oleadginous yeasts to ascertain its importance in controlling the rate of citrate efflux from mitochondria. The following observation were made: Citrate efflux from mitochondria of the oleaginous yeast Condida curuata D, in the presence of l‐malate and pyruvate, was stimulated by adding ATP and reduced by AMP. In the non‐oleaginous yeast, Candida untilis 359, there was very little stimulation of citrate efflux by ATP but it was reduced by AMP. These effects appeared to be generalized as similar results were obtained in an examination of eight further yeasts(seven oleaginous and one non‐oleaginous). The effects of ATP and AMP were not observed in mitochondria whose metabolism had been inhibited by antimycin A and rotrenone indicating the direct reegulation of the citrate translocase was not involved. In C. curuata D, ATP increased the total mitochondrial citrate content and reduced that of 2‐oxogluarate whereas AMP had the reverse effect. In C. untilis 359, AMP had a similar effect but that of ATP was much smaller. To explain these observations the mitochondrial NAD+‐dependent isocitate dehydrogease was studied in a number of yeasts. The enzyme from oleaginous yeasts had a requirment for AMP for activity and was inhibited by ATP. In non‐oleatginous yeasts the enzyme was acrtive in the absence of AMP and increased in activity as the isocitrate concentration increased. The enzyme in C. curuata D was constantly more sensitive to increasing energy charge than that of the non‐oleaginous yeast. These results indicate that the supply of citrate (and hence acety1‐CoA)to the cytosol is controlled by the activity of the intramitochondrial NAD+‐dependent isocitrate dehydrogenase which in turn is regulated by adenine nucleotides. The sensittivity of this enzyme to the ATP/AMP ratio during lipogenesis is therefore an important control in the accumulation of lipid by yeasts. Copyright © 1983, Wiley Blackwell. All rights reserved