A Comparative Study of Citrate Efflux from Mitochondria of Oleaginous and Non‐oleaginous Yeasts

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Abstract
Citrate efflux from mitochondria of 10 different yeasts was investigated. Isolated mitochondria of all the yeasts show the possession of a carrier system for dicarboxylic and tricarboxylic anions, phosphate and pyruvate. The tricarboxylate carrier is also specific for l‐malate in all 10 yeasts. In citrate‐loaded mitochondria, citrate efflux and l‐malate uptake are directly proportional to the concentration of extramitochondrial l‐malate until the translocator becomes saturated (> 8 mM l‐malate in Candida curvata D). Rates of citrate efflux in the presence of l‐malate are linear for a period of 2 min and are approximately 2.5‐times greater in oleaginous than in non‐oleaginous yeasts. The malate‐citrate translocator has a significantly lower Km for l‐malate in oleaginous yeasts (∼ 4.0 mM) than in non‐oleaginous yeasts (∼ 8.4 mM). Both pyruvate and phosphate appear to stimulate the malate‐citrate exchange in mitochondria of C. curvata D. Oleginous yeast mitochondria contain 3—4‐times higher intramitochondrial citrate levels than non‐oleaginous yeast. In all these yeasts the enzymes for synthesis and metabolism of citrate were exclusively mitochondrial, the only exception being the possession of a cytosolic ATP; citrate lyase by oleaginous yeasts. In metabolically active mitochondria, once the malate‐citrate carrie is saturated with l‐malate, efflux of citrate is directly proportional to the concentration of extramitochondrial pyruvate (as acetyl donor). The rates of efflux of newly synthesised citrate from metabolically active mitochondria, in the presence of saturating l‐malate and pyruvate, are 6–8‐times greater in oleaginous yeasts and the citric‐acid‐producing yeast. The inhibition of aconitase by fluorocitrate produced a ninefold increase in citrate efflux from non‐oleaginous yeast mitochondria but only a slight increase in oleaginous yeasts. It is concluded that in fully functional yeast mitochondria, citrate efflux is limited by the amount of intra‐mitochondrial citrate made available to the citrate translocator for exchange.