APPROACHES TO THE ANALYSIS OF SPECIFIC MEMBRANE TRANSPORT 6oi 



formulate hypotheses in accord with as much of the circumstantial evidence as 

 possible, and I think that our aim now must be to try also to develop in vitro 

 membrane systems in which these hypotheses can be put to more crucial tests. 



Frenkel : In the scheme which you showed on your slide glucose 6-phosphatase 

 appears to be present in the cell wall and it is thus difficult to see how it can be 

 solubilized readily. 



Mitchell: I did not say that it was in the cell wall. What I said was that I 

 believed it must be present in the space between the inner margin of the wall, 

 which we know to be impermeable to proteins, and the outer limit of the osmotic 

 barrier component of the plasma membrane. I agree, I think, with the implication 

 of Dr. Frenkel 's remark — that we have to be a little careful about the words 

 describing the cytological structures when we get to such molecular dimensions. 

 The plasma membrane is quite thick — thick enough to be regarded in some respects 

 as a separate phase. There is one functional component of the plasma membrane 

 (we are not sure whether it is in the middle, or near its outer or inner surface), 

 which is mostly a hydrophobic sheet, part probably being protein and part lipid. 

 This hydrophobic sheet is called the osmotic barrier. If we find a catalytic activity 

 exhibited only on the outside of this, the active centre of the catalyst responsible 

 for that activity must be situated somewhere outside the osmotic barrier, or must 

 be in a crevice accessible to the substrate only from the outside. Now, the situation 

 could be that the glucose 6-phosphatase is tucked into or attached to the osmotic 

 barrier, its active centre being accessible to glucose 6-phosphate from outside. 

 When you break the cell, because of the great changes in ionic environment, etc., 

 the enzyme might well become dissociated from the membrane complex and appear 

 as a soluble protein — ^just as we find it. 



DoRFMAN : I wonder if we are not prisoners of our conventional definitions of 

 enzymes and specific proteins. I am thinking of what Dr. Davis said about the 

 analogy of a permease and haemoglobins. In a sense when oxygen is bound it is 

 chemically changed but it is released as the same substance ; in the same way as 

 the transport of glucose as glucose 6-phosphate might occur. I wonder whether we 

 shouldn't think more in terms of protein specificity and less of an enzyme as a 

 catalyst which must bring about a chemical change in the more conventional 

 organic chemical sense ? 



Mitchell: I agree with that remark, but I take exception to the suggestion 

 that we are being blinded by a conventional and old attitude towards enzymes. 

 After all, enzjTne kinetics is a growing subject. I agree with Dr. Dorfman that we 

 need to try to obtain a more biochemical view of transport processes, but I vise the 

 word biochemical to mean conceived in the most up-to-date organic and physical 

 chemical terms. I have no preconceptions as to whether the catalysts of molecule, 

 ion, group, and electron translocation will turn out to be enzymes in the sense that 

 may currently be in use. But I would point out that in the enzyme field we have a 

 number of catalytic carriers, such as the flavoproteins and the haem proteins of the 

 cytochrome system, and one would not say that the cytochrome system caused a 

 chemical change in the electrons which passed through it. Nevertheless, we gen- 

 erally regard these catalysts as part of the overall enzyme system. It is in this sort 

 of context that I am trying to speak. I suppose that if we found proteins with 

 quite new capacities, which were unlike haemoglobin, unlike the catalysts of the 



