EMANUEL EPSTEIN 



105 



Lineweaver and Burk (29) showed that a plot of i/v against i/(S) yields a 

 straight line whose ordinate intercept is i/V, the reciprocal of the maximal 

 velocity, and whose slope equals Kg/V, thus affording a convenient means for 

 determining both constants. They also discussed the effects of inhibitors: in 

 the double reciprocal plot of an experiment in which a competitive inhibitor is 

 used, the slope of the line will increase with no change in intercept. Inhibitors 

 which are not competitive result in increased ordinate intercepts with or with- 



4.5r 



No,m.e/l 



0.5 



0.2 



, 0.3 



0.4 



0.5 



Fig. I. Double reciprocal plots of interference by K {left) and Na with Rb aljsorption by 

 excised barley roots. Rb concentration, S, 2 to 10 mEq/1. Rate of Rb absorption, v, in nig/gm 

 fresh weight in 3 hr. 



out a change in the slope. The reader is referred to the paper by Wilson (45) 

 for a classification of the various types of inhibition. 



To test the validity of the enzyme-kinetic hypothesis of the operation of 

 carriers we measured the rate of ion absorption by excised barley roots as a 

 function of the concentration of the ion being absorbed (the 'substrate ion') 

 and the effect of other ions of the same sign present in the solution ('interfering 

 ior^s'). Figure i presents the results of two typical experiments on Rb absorp- 

 tion. The three features of interest are these: i) the absorption rate varies in 

 response to the Rb concentration in the manner expected; straight lines are 

 obtained when the reciprocal of the rate of absorption is plotted against the 

 reciprocal of the substrate (Rb) concentration, according to Lineweaver and 



