Dietary Regulation of Transketolase Activity in Liver

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Abstract
The effect of varying the dietary intake of thiamine, protein, fat and carbohydrate on the activity of transketolase in liver was studied in weanling rats. Maximum levels of transketolase activity were observed when the diet contained 0.75 mg or more of thiamine hydrochloride per 1000 kcal. It was found that when dietary thiamine is not limiting, the carbohydrate content of the diet appears to determine the level of activity of this enzyme. Of the carbohydrates tested, fructose induced the highest levels of transketolase activity. When either fat or protein was substituted for carbohydrate in the diet, liver transketolase activity decreased. This finding helps to explain why a high fat content in the diet has a thiamine “sparing” action. Like transketolase, hexose monophosphate (HMP) shunt dehydrogenase activity (combined glucose 6-phosphate and 6-phosphogluconate dehydrogenase activities) increased in response to a high carbohydrate diet and decreased with a high fat diet. In contrast to transketolase, however, HMP shunt dehydrogenase activity increased in response to a high protein diet. This divergent response between transketolase and HMP shunt dehydrogenase suggests that the genes for the enzymes of the HMP shunt are not all located on the same operon.