Gluconeogenesis from propionate

Ruminants can produce massive amounts of glucose from propionate. In cows, the cellulose in grass is converted to propionate by bacteria in the rumen. This substrate is then used to generate more than 5 lb glucose each day by the process of gluconeogenesis. 

Gluconeogenesis is the process of converting noncarbohydrates to glucose or glycogen. It is of particular importance when carbohydrate is not available from the diet. Significant substrates are amino acids, lactate, glycerol, and propionate. 

Propionate is not a quantitatively significant gluconeogenic precursor in humans, but it is a major source of glucose in ruminants. It is derived from the catabolism of isolecucine, valine, methionine, and threonine from jd-oxidation of odd-chain fatty acids and from the degradation of the side chain of cholesterol. Propionate enters gluconeogenesis via the TCA cycle after conversion to succinyl-CoA (Chapter 18). 

Butyryl-CoA synthetase is active with acids in the range 3-7C and has been isolated from heart mitochondria. It is a mitochondrial enzyme in other tissues but has not been found in liver where a special propionyl-CoA synthetase has been demonstrated. The latter enzyme may be especially important in ruminants where propionate is formed by rumen microorganisms and is an important substrate for gluconeogenesis. 

---