314 2. MALEATE 



the blood was determined but little activity was found. Thus there was 

 no evidence for the pathway 



Maleate -> fumarate -^ CO 2 



but it is possible that within the tissues the fumarate is so rapidly metab- 

 olized that little escapes into the blood. 



A very interesting pathway for maleate metabolism has recently been 

 established by Taggart et al. (1962). When maleate- 1,4-C^* or maleate- 

 2,3-C^* is injected intraperitoneally in rats, C^^Og appears faster with the 

 former; at 4 hr the C^^Og recovered is 40% and 25%, respectively. The 

 radioactivity in the urine is mainly maleate. If maleate-l,4-C^* is incubated 

 with kidney preparations, C^^Og is released; liver preparations are essen- 

 tially inactive. Neither kidney mitochondria nor supernate are able alone 

 to convert maleate to COg, but together the reaction proceeds readily (see 

 accompanying tabulation). Incubation of maleate- 1,4-C^^ with the supernate 



Preparation % C^Oj recovered in 1 hr 



yields 60-90% of a product shown chromatographically to be D-malate. 

 The D-malate is readily converted to COg by the kidney mitochondria after 

 entering the cycle by means of a NAD-independent D-2-hydroxyacid de- 

 hydrogenase. The supernate enzyme is thus a maleate hydratase, or mal- 

 ease, but presumably different from that isolated from corn. Possibly 

 the renal metabolism of maleate can give rise to some urinary citrate, 

 but it seems unlikely that this is the major factor involved. 



Maleate is utilized in tobacco leaves to a lesser extent than either fum- 

 arate or succinate, 18% of the maleate taken up by the tissue being me- 

 tabolized (Vickery and Palmer, 1956 a). Apparently about half of the 

 metabolized maleate goes to CO2 and the rest to organic acids. From the 

 data in Table 2-5 it can be calculated that an appreciable amount of mal- 

 eate is converted to some acid or acids other than malate or citrate, whereas 

 fumarate is ultimately all converted to citrate. The metabolism of maleate 

 here thus seems to be more complex than a simple isomerization to fum- 

 arate and its subsequent metabolism. 



