File:The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955 (1956) (20604864855).jpg

Title: The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955
Identifier: chemistrymodeofa00wyec (find matches)
Year: 1956 (1950s)
Authors: Wye College; Wain, R. L; Wightman, F. (Frank)
Subjects: Plant regulators; Auxin; Growth (Plants)
Publisher: London, Butterworths Scientific Publications
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Metabolism and mode of action Measurements of sulphydryl disappearance The reactivity of sulphydryl with a-^ unsaturated ketones was first described by Posner (1902). Dickens (1933) established the nature of the reaction of iodoacetic acid with sulphydryls, namely that the iodine was displaced by the sulphur resulting in the formation of a thio-ether. Geiger and Conn (1945) published some preliminary evidence that the activity of some antibiotics might be destroyed by cysteine, and in the same year Cavallito and Haskell (1945) published additional evidence for the reaction of some other lactones with cysteine. In no case, however, has the disappearance of the sulphydryl groups been measured quantitatively. A simple system was used to follow sulphydryl disappearance. One- millilitre aliquots containing various concentrations of growth substances and
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2 3 V- 5 Concn. of iodoacefate exIO ^M Figure 4. Cysteine disappearance with iodoacetate. Volume; 1 • 1 ml. Incubation time: 6 hrs. Reactants adjusted to pH 6-5 ±0-3. Free sulphydryl estimated by nitroprusside assay following dilution with 3-0 ml sat. NaCl. including the sulphydryl compound cysteine were incubated in vacuo at approximately 28°C for 6 hours and the amount of free sulphydryl remaining was then measured by the nitroprusside test of Grunnert and Phillips (1951). We proposed to begin following the disappearance of free sulphydryls in the presence of a model sulphydryl inactivating agent. We selected iodoacetic acid as the model substance and cysteine as the sulphydryl. The disappearance of sulphydryls with iodoacetate is presented in Figure 4. It can be seen that for each mole of iodoacetic acid there was a disappearance of approximately one-tenth of a mole of sulphydryl in 6 hours. Turning to another halogenated acid, Figure 5 shows the results obtained with TIBA. It is evident at once that this compound is at least as active as iodoacetate. For each mole of TIBA added approximately one-half of a mole of sulphydryl disappeared. The apparent reaction between TIBA and cysteine was followed in time as shown in Figure 6. The slow decrease of the sulphydryls in the control was presumably due to auto-oxidation. If TIBA forms aryl-thio-ethers analogous to those formed by iodoacetate, it is interesting to compare the relative —SH activity of halogenated acetic 274

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