538 H. N. CHRISTENSEN et al. 
moving glycine finds itself competing from the outside with escaped methionine 
before it has responded measurably to the rapid movement of methionine out of the 
cells. Suitable manipulation of the relative levels and intervals permits one to 
show the driving of counterflows in some of the less favorable circumstances. 
Although this excursion into the area of exchange processes does not confirm the 
existence of different affinities for exchange from the two sides of the membrane, it 
does again show the dichotomy between the behavior of glycine and AIB, on the 
one hand, and on the other, of valine, cycloleucine and methionine. 
Insummary, although the neutral amino acids yield qualitative evidence of utilizing 
the same reactive site in their transport, when we attempt to place them in an 
affinity sequence on the basis of the rates of their transport, the extents of their 
accumulation, and the magnitudes of their competitive action, serious inconsistencies 
are uncovered, indicating that at least one additional affinity constant must be 
involved, probably for a mediated exodus, perhaps on the modified form of the original 
transport site. 
ACKNOWLEDGEMENT 
The experiments described here were supported in part by a grant (C-2645) from 
the National Cancer Institute, National Institutes of Health, U.S. Public Health 
Service. 
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