310 2. MALEATE 



There are three changes which may be taken as evidence for enzyme in- 

 hibition. (1) The loss of protein nitrogen is stimulated by fumarate and 

 succinate, but is inhibited 33% by maleate. It was believed that this is 

 due to inactivation of proteolytic enzymes. (2) The formation of citrate 

 is again stimulated by fumarate and succinate, but is depressed 57% by 

 maleate. (3) Malate disappeared in the controls, presumably by being 

 transformed mainly to citrate, and fumarate spared this malate by being 

 itself metabolized through malate to citrate, whereas maleate reduced 

 the malate that disappeared. This can hardly be explained on the same 

 basis as fumarate, since so little of the maleate taken up is converted into 

 malate or citrate. Thus an inhibition of malate utilization can be inferred. 

 It should be noted that the leaves cultured in water, fumarate, or succi- 

 nate maintained their turgidity and even increased in weight, whereas 

 those cultured in maleate became flaccid and in 48 hr had lost almost half 

 their weight. The effects on organic acid metabolism by maleate can be 

 satisfactorily explained by a primary effect on the oxidation of pyruvate 

 and of-ketoglutarate, but it is possible that unrecognized inhibitions may 

 be more important. 



The effects of maleate at pH 6 are not basically different from those 

 presented above for pH 5. It is surprising that about 12% more acid is 

 taken up at pH 6. This is opposite to that found for succinate, malate, 

 fumarate, and citrate, and to what might be expected on the basis of the 

 penetration of the less ionized forms. The final maleate concentration in 

 the leaves at pH 6 is actually somewhat less than at pH 5, but this is due 

 mainly to a much greater metabolism of maleate at pH 6. 



Metabolism of Trilaurin in Rat Liver Slices 



Trilaurin labeled in the carboxyl groups incubated with liver slices 

 gives rise to €^^64. The effects of fumarate and maleate are quite different 

 (see accompanying tabulation). Although maleate does not affect the total 



Control Fumarate (5 milf ) Maleate (5 mM) 



101 



CO2 produced, it depresses the appearance of C^^Og. The effects are similar 

 to those produced by malonate, but not as marked (Geyer et al., 1950 a). 

 The action of maleate here could be entirely on the cycle and not neces- 

 sarily on the specific reactions involving oxidation of fatty acids. Nothing 

 is known of the effects of maleate on the enzymes of the fatty acid helix. 



