Aspects of malate metabolism by slices and mitochondria from carrot tissue

Abstract

The endogenous respiration of freshly cut and aerobically circulated carrot slices is completely inhibited by 0.8 and 0.3mM-malonate respectively. Changes in endogenous respiration on aerobic circulation of the slices do not appear to bear any relation to changes in malonate sensitivity. Malate, succinate, a-oxoglutarate and glucose stimulated the respiration of aerobically circulated carrot tissue by 91, 186, 130 and 53% respectively; malonate (20 mM) did not inhibit the respiration with malate but inhibited respiration by 45, 15 and 33 respectively with the other substrates. Malate stimulated the respiration of freshly cut and aerobically circulated tissue preincubated in 20 mM-malonate, but not at higher concentrations; isocitrate did not stimulate the respiration of circulated tissue preincubated in 20 mM-malonate. Malate and glucose stimulated the uptake of malonate by aerobically circulated slices by 59 and 75% respectively; succinate did not affect this uptake. Malonate inhibited by 44% the oxidation of malate by mitochondria isolated from a sample of tissue slices in which malate respiration was insensitive to malonate. Mitochondria prepared in the absence of serum albumin had qO2 values for the oxidation of malate, succinate, pyruvate, isocitrate and [alpha]-oxoglutarate of 34, 29, 16, 12 and 5 respectively. When serum albumin (1%) was added to the blending medium, the corresponding values were 97, 106, 55, 13 and 15 respectively. The addition of 10mM-ADP to mitochondria prepared and incubated in a medium containing albumin increased rates of O2 uptake with malate, succinate, isocitrate and [alpha]-oxoglutarate by 105, 191, 47 and 767% respectively. The mitochondria slowly oxidized glycollate, formate, lactate, glucose 6-phosphate, [alpha]-glycero-phosphate, [beta]-hydroxybutyrate and butyrate. The lactic-dehydrogenase and glucose 6-phosphate-dehydrogenase activities, but not the malic-dehydrogenase activity, were dissociated from the terminal oxidase system on resuspension of the mitochondria. It is concluded that an extramitochondrial metabolism of malate is involved in the malonate-insensitive respiration of carrot-tissue slices.