1 68 H. HOLTER 



Coons that proteins do certainly get inside the cell intact. Now if they go in by 

 pinocytosis then we know also from the work of Simpson and Steinberg in the 

 States, that the breakdown of these proteins takes place by perhaps a reversal of 

 protein synthesis, that is to say not by proteolytic enzymes. As the lysosomes are 

 suicide squads containing proteolytic enzymes, it seems unlikely that the intact 

 proteins are broken down in the lysosomes. On this basis the lysosomes would not 

 be derived from pinocytotic vesicles. 



HoLTER : I quite agree and I think it is one of the main objections to the whole 

 lysosome theory, but I just wanted to present the views that have been suggested. 

 What we really need to do is to reconcile the vacuolar membrane with the evidence 

 for uptake of high molecular svibstances which have been claimed to enter the 

 cytoplasm even without loss of their antigenic properties. To explain this we need 

 to get rid of the membrane barrier somehow. There are people who just assume 

 that the membrane of the pinocytic vesicles disappears, so that their contents are 

 released into the cytoplasm. But clear evidence for this is lacking. In amoebae we 

 have never been able to see the membrane disappear and I understand from Dr. 

 Porter that also the other instances in which such a membrane has been reported 

 to disappear are doubtful on technical grounds. So this is one of the many problems 

 that are still open. 



Davis: I would like to mention an observation which may possibly throw light 

 on this problem. At Harvard Medical School Manfred Karnovsky has been study- 

 ing the metabolic effects of phagocytosis, and has found that the process of engulfing 

 particles markedly stimulates the rate of incorporation of phosphate into phos- 

 pholipid. This suggests that the material of the membrane is turning over. 



HoLTER : Yes, I know that work, and I would also like to mention that, if I 

 remember correctly, the author regards the resynthesis of membrane as the limiting 

 factor for the duration of the phagocytic activity. Furthermore, this paper is the 

 only one I know of where the energetics of such processes have been considered. 

 It is work of that kind that we badly need also in pinocytosis investigations. 



LooMis: I was wondering if the disappearance of the nuclear membrane during 

 mitosis casts any light on how membranes can disappear and reappear. 



Holter: I don't know. Do you ? 



Porter: Well, the observations available suggest that it simply fragments into 

 a lot of smaller vesicles without any decrease or increase in the total area of the 

 membrane involved. 



