EFFECTS ON THE ENDOGENOUS RESPIRATION 181 



that the inhibition developes over 1-2 hr. A very similar time course was 

 observed in barley roots by Laties (1949 a). After the addition of 10 mM 

 malonate, the respiration drops linearly for 60-90 min and then becomes 

 relatively constant at about 40%. However, over the next 5 hr, the inhi- 

 bition lessens somewhat. This was postulated to be due to increasing suc- 

 cinate concentration, but actually such changes in succinate take place 

 much more rapidly in most cases. It must be admitted that in spinach 

 leaves the maximal succinate accumulation occurs at 4 hr (Laties, 1949 b), 

 so that this explanation can by no means be eliminated. The inhibition 

 of the spinach leaf respiration by malonate is greater at 6 hr than at 3 hr 

 but not enough data are available to correlate the changes in inhibition 

 with succinate levels. Malonate requires about 1 hr to produce its maximal 

 inhibition of sea-urchin egg homogenate respiration (Yeas, 1950). This is 

 the only report of such a slowly developing inhibition in subcellular pre- 

 parations and no explanation is evident. Another situation seems to exist 

 in bovine kidney culture cells, where the inhibition by 100 mM malonate 

 slowly increases from 23% at 1 hr to 35% at 5 hr (Polatnick and Bachrach, 

 1960). It is more likely here that secondary changes are responsible. 



Definite decrease in the respiratory inhibition of pigeon brain dispersions 

 (probably similar to homogenates) with time was reported by Banga et al. 

 (1939). The inhibition is 41.7% at 10 min, 29.5% at 30 min, and 20.5% 

 at 50 min. Since the malonate concentration was 24 mM, it seems unlikely 

 that metabolism of malonate could have reduced its concentration signifi- 

 cantly. Adaptive changes in homogenates are improbable and sufficient 

 accumulation of succinate from the relatively limited substrate supply is 

 scarcely possible. Besides, the inhibition of pyruvate oxidation increased 

 over this interval. Sometimes changes in tissue metabolism occur during 

 the course of an experiment independent! j^ of malonate action. When mal- 

 onate is added to human brain slices immediately, the endogenous respira- 

 tion is inhibited around 25% at 5 mM, but if malonate is added after 

 90 min incubation of the slices, there is no inhibition (Elliott and Suther- 

 land, 1952). The role of succinate oxidase in the respiration must change 

 as a result of the slicing or the abnormal medium. 



The inhibition of the respiration of rat ventricle slices by malonate 

 5-20 mM follows a more complex course (Webb et al., 1949). Following 

 an initial inhibition, the respiration rises for approximately 1 hr and then 

 begins to fall again. There is thus a maximum or hump in the respiration 

 curve. After 1 hr, the respiratory level with malonate is higher than in 

 the controls. The reversal of the inhibition is inhibited by fluoride, which 

 would indicate that the hump is due to augmented glucose oxidation or to 

 the metabolism of malonate. Calcium is also necessary for the typical re- 

 sponse to malonate. Very complex effects of malonate were also found by 

 Turner and Hanly (1947) and Hanly et al. (1952) in carrot slices. The var- 



