Chemical structure and biological activity 



actually bound, an assumption that finds good support in the studies of auxin 

 uptake in plant tissues made by Sutter (1944) and by Reinhold (1954). 

 Also, experiments with labelled 2:4-D (Hanson and Bonner, 1955) do 

 seemingly indicate that the concentration of readily exchangeable regulator 

 in the tissue is comparable to that of the surrounding solution. Qjuite 

 generally we then find : 



M MIK 



a = -— , or a — 



where a is the fraction of places in an adsorption system which are occupied 

 by the regulator, M the molar concentration of this regulator, and K the 

 dissociation constant of the receptor-regulator complex. For a ^ 0-5 we get 

 K = M. Upon addition of another regulator, for example an anti-auxin 

 with the constant K' at the concentration M', we find the a-value for the 

 first substance diminished to 



MIK 



Cf. 



l-^MjK + M'jK' 



Making further the rather bold assumption that the growth results, for 

 example root-growth inhibition, are proportional to a, action curves may be 

 constructed which resemble the empirical cvirves very closely. The effect 

 of an antagonist in displacing the root-growth inhibition curve to higher 

 auxin concentrations without any fundamental change in its form, is also in 

 good agreement with expectation (Aberg, 1951; Aberg and Jonsson, 1955; 

 Hellstrom, 1953). Some complications arise, however, from the presence of 

 native auxin in the roots and from the non-specific toxic effects of higher 

 concentrations of the antagonist. The action curve of a pure anti-auxin may 

 be deduced as resulting from an adsorption in two patterns (competition 

 with the native auxin, toxic effect at higher concentrations), the actual 

 growth being determined by the product of both influences (Hellstrom, 1953). 

 The Michaelis-Menten formula may be written as follows: 



V [^] M 



V K+[S] K-\-M 



where v is the reaction velocity, V the maximum reaction velocity, \S'\ or M 

 the substrate concentration at equilibrium, and K the dissociation constant 

 of the enzyme-substrate complex. Now, v is thought to be proportional to 

 the amount of this complex, and vjV \% thus equal to a. If we now speak of 

 the plasmatic receptor instead of the enzyme and the growth regulator 

 instead of the enzyme substrate, this type of treatment appears to be identical 

 with the former one. The assumption that a very small proportion of the 

 regulator is actually bound, is made by McRae et al. (1952, 1953) by putting 

 [S^ equal to the concentration of added regulator. 



Though calculations with help of the methods indicated above may often 

 give results which are in good agreement with a limited series of experimental 

 data, difficulties certainly arise when a more extensive set of results have to be 

 treated. This is perhaps not surprising when due respect is paid to the 

 complexity of the situation. The proportionality between the amount of 



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