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DISCUSSION 



Chance : I have a question on the function of the pyridine nucleotide 

 cytochrome c reductase in the endoplasmic reticulum. Assuming that 

 there is cytochrome c in the cytoplasm, what is the electron acceptor for 

 cytochrome c reductase which would enable it to transfer electrons and 

 thus to act as an ion pump mechanism? While there is surely cytochrome 

 fej in the microsomal particle, there does not appear to be any oxidant 

 for it. Therefore, while one could get a "substrate amount" of electron 

 transfer, how can there be any continuous electron transfer in the 

 endoplasmic reticulum? This point also refers to Palade's ideas about 

 the endoplasmic reticulum of muscle. It is a question of whether this 

 pyridine nucleotide cytochrome c reductase is in the endoplasmic 

 reticulum of the muscle. 



Siekevitz : No one knows about the electron acceptor for DPNH in the 

 endoplasmic reticulum. We are working now on the question of the 

 presence of a DPNH-cytochrome c reductase in muscle microsomes, and 

 Dr. Luck in our laboratory has recently found this enzyme to be active 

 in heart muscle microsomes. 



Lynen: In the experiment where you added hexokinase to mito- 

 chondria and spun it down, you found that the sum of the activities of 

 mitochondria and supernatant was much higher than the original 

 hexokinase activity. 



Siekevitz: Yes, and one of the causes might be dampened enzyme 

 activity due to product inhibition. Inhibition of hexokinase by the 

 product, glucose-6-phosphate, has been reported (Crane, R. K., and Sols, 

 A. (1953). J. bioL Chem., 203, 273). When we had to use a large amount 

 of mitochondria in order to get the mitochondrial nucleotides, we also 

 brought down with it some of the microsomes which had glucose-6-phos- 

 phatase activity. We demonstrated that the mitochondrial preparation 

 had some of this glucose-6-phosphatase activity ; so the activation might 

 be due to a relieving of the inhibition caused by glucose-6-phosphate. 



Rocker: Did you use yeast hexokinase in these experiments? 



Siekevitz: Yes. 



