Quantitative aspects of the receptor binding of cytokinin agonists and antagonists

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Abstract
Congeneric 4-anilino- and 4-(alkylamino)-2-methylpyrrolo[2,3-d]pyrimidines showed cytokinin and anticytokinin activities [measured in Nicotiana tabacum callus] depending on the structure of their 4-substituents, and the antagonistic nature of the latter was established kinetically. The effect of the substituent on these activities was analyzed quantitatively by using physicochemical parameters and regression analysis to give a single, common equation for both the agonists and antagonists. Evidently the maximum width of the N4 substituents is an important factor both for binding to the receptor, thus the extent of activity, and for the quality of activity, agonistic or antagonistic. The electron-withdrawing effect and hydrophobicity of the substituents further enhance binding and activity, irrespective of the quality of the activity. Evidently in hormonal action, agonist binding causes a conformational change of an otherwise inactive receptor to the active form and antagonists are species that bind similarly to the receptor but do not cause the effective conformational change.