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INVITED DISCUSSION 
SOME REMARKS ON ACTIVE TRANSPORT 
AND EXCHANGE DIFFUSION OF AMINO ACIDS 
IN PHREICH CERES 
ERICH HEINZ 
Johann Wolfgang Goethe-Universitadt, Frankfurt a. Main (Germany) 
Transport of amino acids in Ehrlich mouse-ascites tumor cells is associated to a 
high extent with exchange diffusion. As shown in Fig. 1 preloading these cells with 
cold glycine or related amino acids strongly accelerates the influx of glycine? ? 
We assume that this phenomenon, which represents a true one-to-one exchange, 
is related to similar ones later reported for other systems, e.g. by PARK et al.°, by 

Time (min) 
Fig. 1. Relative uptake of [1-C]glycine after preloading cells with unlabeled glycine. Solid 
lines, distribution ratios of radioactivity between cells and medium. Dotted lines, distribution 
ratios of total glycine. The circles and squares refer to preloading experiments and controls, 
respectively. Krebs’—Ringer—bicarbonate solution at 36° was used as medium. (From HEINZ AND 
WatsH?, with permission). 
RosENBERG ef al. under the term “uphill transport induced by counter flow’”’ 
and by HEINz AND DurBIN®. With Ehrlich nouse-ascites tumor cells there is evidence, 
that this exchange diffusion refers to the same carrier mechanism as does the active 
transport. This follows from the wide agreement of both processes with respect to 
substrate specificity?» ® and to susceptibility to lipotropic inhibitors’. With Kromp- 
HARDT we provided further evidence analyzing the glycine influx in terms of the 
Michaelis-Menten equation with and without preloading*. Preloading increases the 
maximum velocity whereas the Michaelis constant remains the same (Fig. 2). 
Exchange diffusion indicates the degree of reversibility and hence the thermo- 
References p. 544 
