182 1. MALONATE 



iation of the inhibition depends on the pH. At pH 4, the inhibition de- 

 velops over 1 hr and remains constant, but at higher pH's the inhibition 

 may disappear or only stimulation may be seen. The value of these interest- 

 ing experiments is greatly reduced by the unaccountable use of potassium 

 malonate rather than the sodium salt. Potassium at 50 mM (which is the 

 concentration of malonate generally used by them) stimulates the respira- 

 tion and alters its character, so that aU the results must be the summation 

 of two usually opposing actions. This illustrates how the incorrect choice 

 of an inhibitor salt can vitiate the results of an otherwise excellent in- 

 vestigation. 



Most of the work on the malonate inhibition of endogenous respiration 

 has been done without regard for possible alterations in the inhibition 

 with time. The inhibitions have simply been determined over an arbitrary 

 interval. Inasmuch as changes in the inhibition by malonate occur quite 

 frequently, it is likely that over all inhibitions, such as are presented in 

 Table 1-26, are often mean values and do not reflect either the initial inhi- 

 bition or the maximal inhibition. As was pointed out in Chapter 1-12, the 

 value of many studies on inhibitors would be increased by determinations 

 of the variation of the inhibitions with time. 



Effects of Different Conditions on the Inhibition of Endogenous Respiration 



One of the most important variables affecting the response of the endog- 

 enous respiration to malonate is the age of the tissue, particularly as it 

 relates to the stage of development or the interval between the preparation 

 of the tissue and the experimental testing. The respiration of plant tissues 

 usually becomes more sensitive to malonate with time. This indicates a 

 progressive change in the metabolic pattern in the direction of a greater 

 participation of the cycle. The changes in the inhibition during malonate 

 inhibition, discussed in the previous section, can be due to the effects of 

 the malonate or to inherent metabolic alterations. It is thus important in 

 such studies to determine both the changing inhibition in the presence of 

 malonate and the changing susceptibility as malonate is added at various 

 intervals. The malonate inhibition rises with time in the Avena coleoptile 

 (Bonner, 1948), rose petals (Siegelman et al., 1958), chicory root slices (La- 

 ties, 1959 a), Arum spadix slices (Simon, 1959), and potato tuber slices 

 (Romberger and Norton, 1961). These changes are usually associated with 

 an increase in the total uninhibited respiration. For example, in potato 

 slices there is a 4-fold rise in the respiration during incubation for 30 hr; 

 the malonate resistant fraction doubles and the malonate-sensitive fraction 

 increases 10- to 15-fold. Carrot slice inhibition by malonate, on the other 

 hand, decreases steadily up to 376 hr after cutting the sections (Hanly 

 et al., 1952), although the uninhibited respiration first rises and then falls. 

 The results obtained with animal tissues are less striking and more variable. 



