22 



1. MALONATE 



Competition between succinate and malonate has been claimed to occur in 

 the following: rat liver homogenates (Potter and DuBois, 1943), oyster 

 muscle homogenates (Humphrey, 1947), oyster egg homogenates (Cleland, 

 1949), carrot root (Hanly et al, 1952), yeast (Krebs et al, 1952), pig heart 

 particulates (Thorn, 1953), cockroach muscle homogenates (Harvey and 

 Beck, 1953), the trypanosome Crithidia (Hunter, 1960), and the soluble 

 succinate dehydrogenase from heart (Keilin and King, 1960). In all cases, 

 the inhibition produced by a certain malonate concentration is reduced by 

 increasing the succinate concentration, but a quantitative analysis of the 

 data has been seldom carried out. Most of these studies were made with 

 the usual assay methods, but competition has recently been shown by meas- 

 uring the reductions in the electron spin resonance signals produced by 

 malonate at different succinate concentrations (Commoner and Hollocher, 

 1960). 



Fig. 1-3. A single-curve plot for the inhibition of 



cockroach muscle succinate oxidase by malonate at 



0.33 mM. Ki = 0.105 mM and KJK^ = 275. (Data 



from Harvey and Beck, 1953). 



Single-curve plots (type F, see Chapter 1-5) were made for two of the 

 inhibitions mentioned above (Figs. 1-3 and 1-4). In the case of Crithidia, 

 Hunter showed competition with a 1/'U-1/(S) plot and the single-curve plot 

 confirms this, K^ having the value of 0.22 mM and K^^JK^ calculated from 

 the slope a value of 53. The results with cockroach muscle likewise fall 

 roughly on a straight line, giving K, as 0.11 mM and KJK^ as 275, values 

 differing somewhat from those calculated by Harvey and Beck using a 



