308 2. MALE ATE 



were obtained in vivo (see page 319). The reactions in liver mitochondria 

 may be written as: 



a-Ketoglutarate + NAD+ -> succinate + COg + NADH + H+ 

 a-Ketoglutarate + NH4+ + NADH -> glutamate + NAD+ 



2 a-Ketoglutarate + NH4+ -> succinate + glutamate + CO2 + H+ 



the reductive amination of a-ketogkitarate utilizing the NADH formed in 

 the oxidation of the remaining a-ketoglutarate, since there are approx- 

 imately 2 moles of a-ketoglutarate removed for each mole of amino acid 

 formed. In the kidney around 5 moles of a-ketoglutarate disappear for 

 each mole of amino acid formed. It is difficult to decide which reaction is 

 inhibited by maleate, but the relative constancy of the a-ketoglutarate/ 

 amino acid ratio led Rogulski et al. to assume that the primary effect is 

 on the oxidation of a-ketoglutarate. Certainly the or-ketoglutarate oxi- 

 dation in kidney homogenates is quite sensitive to maleate, whereas that 

 in liver is not (Angielski and Rogulski, 1962). The moderate rise in the ratio 

 could be explained by a less potent inhibition of the amination reaction, 

 but also by less efficiency in the utilization of NADH when little is formed. 

 In any event, these results indicate a remarkable difference between liver 

 and kidney with respect to the response to maleate, and this has impor- 

 tant bearing on the results obtained in whole animals. 



Organic Acid Metabolism In Tobacco Leaves 



Perhaps the most thorough and interesting study of the complex effects 

 of maleate on metabolism is that of Vickery and Palmer (1956 a), using 

 tobacco leaves cultured with maleate, fumarate, or succinate at 200 mM 

 and pH 5 for 48 hr. It was known that fumarate is well utilized by these 

 leaves and that maleate often increases the respiration of plant tissues; 

 thus an investigation was made into the question of whether maleate is 

 utilized and whether it exhibits inhibitory effects on metabolism. The 

 effects are summarized in Table 2-5. It is evident that maleate is meta- 

 bolized to some extent, but this will be discussed in a later section (page 

 314), and here we shall consider only the evidence for an inhibitory ac- 

 tion on certain phases of the leaf metabolism. It is first necessary to cal- 

 culate the final concentration of maleate in the leaf water: the leaves 

 initially weighed 1000 g and lost 489 g during 48 hr, so that final weight 

 was 511 g; total solids were 122 g, so that 389 g of water were present; 

 the leaves contained 239 meq of maleate, so the concentration is 310 mM. 

 If the uptake was linear, as it is for fumarate, the concentration must have 

 been over 100 mM for 24 hr or more, so that there would appear to be 

 both ample time and concentration for inhibition to be exerted. 



