MALEIC HYDRAZIDE 327 



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(I) (II) (III) (IV) 



dicating that structures (II) and (IV) are important (at physiological pH's 

 actually the ionized species would be dominant) (Miller and White, 1956). 

 (3) The C = C bond in maleic hydrazide does not behave as a typical olefinic 

 bond, so it is likely that some aromatization of the ring occurs, again 

 explained by structures (II) and (IV). Structure (III) does not appear 

 to be important. Maleic hydrazide is soluble in water to the extent of 

 0.2% (about 18 mM), but as the diethanolamine salt, in which form it 

 is usually applied commercially, it is soluble to at least 2%. 



The rapid reaction of maleimide with thiols might lead one to expect 

 maleic hydrazide also to be reactive, but apparently the aromatization of 

 the ring markedly reduces the reactivity. When 5 mM maleic hydrazide 

 is incubated with 0.5 mM glutathione for 5 hr at room temperature, there 

 is no detectable disappearance of SH groups (Leopold and Price, 1957). 

 This is true in the entire pH range 4-7. Furthermore, incubation of maleic 

 hydrazide with thiolacetate, cysteine, or glutathione at 37o and pH 7 

 for 4 days does not lead to significant loss of SH groups (Weller et al., 

 1957). No alteration of the SH groups in radish leaf homogenates by 

 maleic hydrazide was observed. Glutathione does not protect phosphorylase 

 against inhibition by maleic hydrazide, indicating that no reaction with 

 glutathione occurs (Hughes and Spragg, 1958). Tests with bromine and per- 

 manganate showed the presence of an olefinic bond in maleic hydrazide, 

 and indicated that the C = C bond retains a certain high reactivity charac- 

 teristic of maleate derivatives, but that this reactivity is not sufficient to 

 allow addition of the SH group, at least in low molecular weight thiols. 

 However, Hughes and Spragg (1958) found that around 60% of the SH 

 groups of /^-amylase disappeared after incubation with 4 mM maleic 

 hydrazide, so that possibly certain SH groups on proteins may react more 

 readily than the simple thiols. The statement by Hughes and Spragg, 

 "A reaction could take place between two SH groups and one molecule 

 of maleic hydrazide in such a manner that two protein chains become linked 

 irreversibly" to explain certain mitotic effects, is open to question. 



General Effects on Plant Structure and Growth 



Maleic hydrazide is able to inhibit the germination of seeds, suppress 

 the growth of roots and terminal shoots, and retard bud and floral develop- 



