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Discussion 



Lowenstein: Dr. Chappell's first slide showed that isocitrate was oxidized as 

 rapidly as glutamate. Some years ago Dr. Lardy showed that isocitrate is oxidized 

 much more slowly than glutamate. Is this a question of the conditions that you use ? 



Chappell : Dr. Lardy was using Warburg manometers and I am using an 

 oxygen electrode. 



Lowenstein : We used an oxygen electrode and obtained the same results. 



Ch.^ppell: What was your level of phosphate, M 25 phosphate ? 



Lowenstein: We used somewhat lower concentration, plus hexokinase and 

 glucose. 



Chappell: M 25 phosphate is inhibitory unless you add catalytic amounts of 

 malate. I think the point of action of the phosphate is on fumarase because at low 

 fumarate concentrations phosphate does inhibit fumarase quite markedly, thus it 

 is acting as malonate would, or Lewisite, and preventing the catalyst from getting 

 into its position to catalyse more citrate oxidation. 



Lowenstein: You think it is the concentration of orthophosphate which is 

 critical ? 



Ch.appell : I am sure it is, you can show it very easily. 



Lardy : I should like to point out that De Luca and Steenbock have found very 

 striking differences between the rate of oxidation of isocitrate in rats with and 

 without vitamin D and they think that this is a controlling factor determining the 

 levels of citrate in tissues ; there is a considerable increase in the citrate content of 

 tissues in the animals getting the normal supplement of vitamin D. I wonder 

 whether your experiments could not be investigated using vitamin D-deficient 

 rats to see what effects vou would get. 



