Malonate and carrot root respiration.

Following a review of earlier work with malonate as an enzyme and respiration inhibitor, direct evidence is provided of the existence in carrot root tissue of cytochrome oxidase and succinic dehydrogenase (S.D.). Malonate is clearly effective as an inhibitor of carrot root respiration only at low pH. Its effects at higher pH are, however, fully described and discussed. It is postulated that in this tissue a significant part of the respiration is mediated by enzyme systems not inhibited by malonate, KCN, or CO; that the remainder, whose activity is varied by wounding and aging, and by ionic exchange and uptake, involves an organic acid cycle of the Krebs type. The effects at low pH and low concentration of malonate (O.005-0.02M) may be explained as due to inhibition of succinic dehydrogenase only; under these conditions self reversal of inhibition, and reversal by addition of succinate, are both possible. At higher concentrations ( O.04-0.05M ) and low pH, malonate is assumed to inhibit not only S.D. but other enzymes concerned in pyruvate oxidation; this explains the lack of self reversal, lack of reversal by added succinate, and the failure to demonstrate accumulation of succinate in poisoned tissue; under these conditions, when inhibition is to the basal level, the RQ. is high, presumably because pyruvate is diverted to form fermentation products.