Activation of succinate dehydrogenase in isolated mitochondria by noradrenaline.
Activation of succinate dehydrogenase (EC 1.3.99.1) [9002-02-2] was obtained on preincubating rat liver mitochondria with noradrenaline (I) [51-41-2] and 3,4-dihydroxybenzoate [99-50-3], a compd. which could be derived from the amine. The KA for 3,4-dihydroxybenzoate (6.3 \times 10-4M) was one order of magnitude less than that of I (6.2 \times 10-3M). The activation was of the V type, with no change in the Km for succinate, the 1st of this type among the activators of succinate dehydrogenase. Addn. of I to the assay medium inhibited the enzyme activity but the kinetics of inhibition were different from those of the other known classes of activators of succinate dehydrogenase, including 3,4-dihydroxybenzoate. Experiments on washing activated mitochondria prepration indicated strong binding of both the activators, and the activation was reversed only after preincubation with succinate followed by 2 washings with the homogenizing medium. Mitochondria isolated from rat tissues, viz. heart, kidney, and brown adipose tissue, also showed activation with I. Liver mitochondria from cold-acclimated rats showed a sensitization effect giving the same max. activation at a lower concentration of I.