. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Metabolism and mode of action Veldstra and Havinga (1943) suggested that unsaturated lactones such as coumarin were sulphydryl inactivating agents. We have found in fact that coumarin incubated with cysteine brought about a disappearance of free sulphydryl as shown in Figure 7. Each mole of coumarin brought about the disappearance of about one-third of a mole of sulphydryl. Turning to 2:4-D as a model auxin, it
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Metabolism and mode of action Veldstra and Havinga (1943) suggested that unsaturated lactones such as coumarin were sulphydryl inactivating agents. We have found in fact that coumarin incubated with cysteine brought about a disappearance of free sulphydryl as shown in Figure 7. Each mole of coumarin brought about the disappearance of about one-third of a mole of sulphydryl. Turning to 2:4-D as a model auxin, it would be very interesting to know whether such a sulphydryl reaction might occur with this compound, par- ticularly in view of the suggestion that auxin attaches to a cysteinyl group. It can be seen in this figure that 2:4-D gave only very small disappearance of sulphydryl. It would seem, therefore, that this auxin does not readily form a thiol bond under the conditions it-x10''^V\ 2 V- Concn. of growth substances e xio'^w Figure 7. Cysteine disappearance with 2:4-1), maleic hydrazide, and coumarin. Conditions as in Figure 4. Maleic hydrazide was also only weakly active at best in causing sulphydryl disappearance. Another compound it would be interesting to examine for thiol reactivity is 2:4-dichloroanisole. Bonner (1949) first suggested this compound as a competitive inhibitor of auxin on the basis that it might satisfy the ring requirement for auxin (presumably by sulphydryl attachment) without satisfying the acid requirement. Consequently, it would be an anti-auxin by competitive inhibition. In Figure 8, it appears that 2:4-dichloroanisole may be quite effective in bringing about the disappearance of free sulphydryl, and this is in contrast to the relative inactivity of 2:4-D, and incidentally, of 2:4-dichlorophenol. Due to solubility limitadons of the dichloroanisole, a small amount of ether was added to this experiment, and since this solvent interferes somewhat
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