Acetone Metabolism 245 



than does formate. The Neuberger group has shown that other 1 -carbon 

 compounds are also incorporated into serine. There is no doubt that 

 formaldehyde has to be considered. I think that the results of pro- 

 pionic acid fermentation are especially significant, because in this case 

 we know that formaldehyde itself is actually formed. 



Rittenberg: Dr. Price in my laboratory has been interested in the 

 oxidative route taken by acetone, especially with regard to its utiliza- 

 tion for cholesterol synthesis. In tissue slice experiments he has found 

 by use of the washing-out procedure that pyruvic acid does not diminish 

 the utilization of methyl-labelled acetone for cholesterol formation. 

 This presumably means that pyruvate is not on the pathway from 

 acetone to acetate. Curiously enough, methyl glyoxal does diminish 

 the utilization of acetone for cholesterol synthesis. We are not sure 

 now whether we are dealing with a true intermediate or whether methyl 

 glyoxal by some indirect method interferes with this conversion. 



Wood: When I put pyruvate in quotes, that implies any Cg precursor 

 of glycogen: methyl glyoxal, lactic acid. I think there is a fair chance 

 that methyl glyoxal might be formed. 



Block: If I remember correctly, the isotope concentrations in 

 carbons 1 and 6 were about 20 per cent higher than in carbons 2 and 5 

 after methyl-labelled pyruvate or lactate. Would you interpret this 

 to indicate that the direct conversion of pyruvate to glucose is a rela- 

 tively minor pathway, and that the major part of pyruvate is meta- 

 bolized by way of the Krebs cycle, at least in the liver? 



Wood: Yes. I would interpret it that way, except I don't know 

 whether the pyruvate is metabolized via the entire cycle or just shuttles 

 back and forth very rapidly to C4 carboxylic acids. The fixation reaction 

 is much faster than our previous results indicated. Dr. Utter now has 

 found in the carboxylation reaction that oxaloacetate is almost com- 

 pletely equilibrated with CO 2 in 10 minutes. Thus the conversion of 

 CO2 and pyruvate to oxaloacetate is not a slow reaction. 



