Discussion 47 



sign of pieces of endoplasmic reticulum remaining attached to the 

 nuclear membrane. 



How do you reconcile the system of intracellular membranes with the 

 mitochondrial movements that go on in the living cell? The membranes 

 would seem to limit the regular movements that you get with mito- 

 chondria. 



Siekevitz : If you have a system of channels formed by the reticulum 

 membranes then the mitochondria may just move back and forth 

 between two adjacent channels. 



Bartley : Have you any idea as to what is the force causing the move- 

 ment? 



Siekevitz: Bennett (1956, loc. cit.) has published a theory that the 

 whole membrane system is in motion through the cell. To go back to 

 the nuclear picture, he thinks that if you have membranes of the endo- 

 plasmic reticulum enclosing the nucleus, the nucleoprotein particles 

 which are on the endoplasmic reticulum membrane might come from the 

 RNA elaborated by the nucleus and be deposited on the membranes 

 lining the nucleus, and then by the actual movement of the membrane 

 become the nucleoprotein particles of the endoplasmic reticulum 

 (microsome) fraction. There is no evidence for this, but there is one 

 piece of evidence against it, and that is that the RNA of these particles 

 has a very low turnover as compared to the RNA you get from the 

 nuclei. 



Aldridge: On the hexokinase question, we found that with mito- 

 chondria we can stimulate fourfold by the addition of hexokinase, but 

 if we titrate with minimal amounts of hexokinase up to maximum 

 stimulation and also measure the inorganic phosphate uptake under 

 those conditions, we find that the amount of hexokinase we add is not 

 sufficient to account for the phosphate uptake; there is a threefold 

 differentiation. 



Dr. Siekevitz, you have described an experiment in which the nucleo- 

 tide levels were measured upon adding hexokinase. I have recently 

 carried out a similar experiment with the addition of DNP. The liver 

 mitochondria used have excellent stability, for the addition of potato 

 apyrase to them after they have been metabolizing pyruvate at 37° at 

 the slow unstimulated rate for 2 hours still produces a fourfold stimu- 

 lation. 



Titration with DNP increases oxygen uptake until a plateau is reached 

 at 1 X 10 -^M (4-1 -fold stimulation). The concentration of ATP is main- 

 tained right up to maximal stimulation of oxygen uptake and only falls 

 and ADP increases when more DNP is added (Fig. 1 p. 48). This probably 

 has physiological significance and also explains why it is so difficult to 

 detect a fall in ATP concentrations in the tissues of animals after non- 

 lethal doses of DNP. 



Siekevitz: That agrees with some results we had, with addition of 

 DNP and hexokinase, on the ability of the mitochondria to maintain the 

 ATP at maximum stimulation. 



Potter: This is almost a perfect duplication of our data. Our inter- 

 pretation was that here we were measuring the total nucleotides. We 



