Failure of Maleic Hydrazide to Act as a Sulfhydryl or Carbonyl Reagent

Abstract

A wide variety of molecular mechanisms have been proposed to explain the actions of the synthetic plant growth regulator maleic hydrazide (3). One of the most prominent of these theories holds that MH2 acts as a sulfhydryl reagent (7), and more recently it has been suggested that MH acts as a carbonyl reagent (12). The idea that MH might react with sulfhydryl groups was supported by reports that it can inhibit respiration in several plant tissues and can block several sulfhydryl enzymes (13). Several subsequent studies, however, mM MH (-U--), 5 mM L-cysteine (-0-), a mixture of MH and cysteine at these concentrations (allowed to stand at 25 C for about 30 min) (--0--), and the sum of the absorption spectra of MH and cysteine taken separately (--0--). All of these solutions and the reference buffer contained 20 mm tris-HCl at pH 7.0. These absorption spectra were measured on a Cary recording spectrophotometer in cells with a 1-cm path length.indicate a lack of effect on both respiration and many sulfhydryl enzymes (1,3,4,10,13). Furthermore, MH does not decrease the number of thiol groups in radish leaf homogenates (13), and attempts to prevent MH from inhibiting root growth with cysteine were unsuccessful (8) UV spectral studies to determine if MH and sulfhydryl or carbonyl compounds interact in such a way as to cause spectral changes. This would be expected if they were linked covalently, because a double bond would probably be lost or at least the distribution valency electrons would be changed (5, 11). (b) Using paper chromatography, a search for possible derivatives formed through reaction with sulfhydryl or carbonyl compounds. Figure 1 shows the UV absorption spectrum for 0.1 mm MH (recrystallized seven times from hot water) in 20 mM tris, pH 7. Maximal absorption occurs at about 328 nm and minimal at about 265 nm. This corresponds well with spectra reported for MH in sodium pyrophosphate buffer at pH 8.4 (6). Figure 1 also demonstrates that a mixture of freshly made 5 mM cysteine (chosen because it is the thiol suspected to react with MH and it is less hindered stericly than cysteine in protein) and 0.1 mm MH in 20 mm tris, pH 7 (selected to approximate a physiological pH) absorbed UV light to the same extent as the sum of the absorbances of the MH and cysteine separately. Similar results were obtained with 0.2 mm MH and 6 mm cysteine. Thus cysteine and MH do not interact in a way which changes their UV absorption. As expected, a similar mixture of 2 mm cysteine and 1 mm iodoacetate, a well known sulfhydryl reagent, undergoes considerable change in UV absorption; relative to the sum of the individual spectra, the mixture absorbs less from about 249 to about 320 nm and more below 249 nm.Pyruvate was selected as the carbonyl compound to test for reaction with MH. because it was used to reverse MH inhibition of monoamine oxidase, a result which led to the suggestion that MH acts as a carbonyl reagent (12). When 5 mM pyruvate and 0.2 mm MH are mixed as above in 20 mM tris, pH 7, the mixture again absorbs UV li...