KINETIC APPROACH TO TRANSPORT 



dA 2 



dt 



= Y + K B (A l -A 2 ) (7) 



where Y is the rate of the saturable component. Under the assump- 

 tion that A x had been kept constant by the use of a large volume of 

 suspending solution, and that the rate of the saturable component 

 Y is independent of time and of the intracellular concentration, 

 A 2 , Equation (7) can be integrated to give 



A 2 F(l - e-*D<) i 



IT - -kT~ .47 + (1 - e D,) (8) 



Now, at high extracellular concentrations, the saturable process 

 will be saturated, and Y will be very nearly constant. Therefore, 

 the last term (1 - e - a 'd< ) will be the intercept of a straight line 

 relating \/A x and the distribution ratio A 2 /Ai at sufficiently high ex- 

 tracellular concentrations. By evaluating this term (1 — e _A 'o') 

 graphically, X D , the apparent difTuson constant, was evaluated and 

 introduced into Equation (8). The value of Y, the rate of the satu- 

 rable component, could then be determined, and was found indeed 

 to be nearly independent of time during the interval of study. As 

 the concentration increased from 1 to 20 mM, the saturable com- 

 ponent of the entry rate in the presence of insulin fell from 99 to 

 61 per cent of the total entry rate (Figure 19). The saturable com- 

 ponent was shown further to follow Michaelis-Alenten kinetics. In 

 the absence of insulin, similar relationships held, except that K m 

 appeared to be 10 times as large, whereas V m and Kt> were not meas- 

 urably changed. 



Inhibition 



Transport inhibitors, such as 2,4-dinitrophenol, cyanide, azide, 

 and iodoacetate, interfere with the energy supply for transport, and 

 probably do not react at all in the transport process. A more inter- 

 esting type of inhibitor is one with an affinity for the same site at 

 which the solute is accepted in a mediated transport, and with no 

 other action on the transport system. Both among the amino acids 

 and among the sugars (and apparently also among the lower fatty 

 acids, anions, the purines, and other classes), we find whole families 

 of solutes reacting, each with their specific transport site, with affini- 

 ties falling in sequences that permit informative study. 



5 1 



