Effect of Thiamine Deficiency on Gluconeogenesis and Transketolase Activity in Isolated Hepatocytes

Abstract

Abstract

The effect of a 4-week thiamine deficiency on gluconeogenesis from xylitol, lactate, pyruvate, dihydroxyacetone, and fructose in isolated rat hepatocytes was investigated. Xylitol is metabolized via transketolase, a thiamine-requiring enzyme, whereas the remaining substrates are converted to glucose via enzymes which do not require thiamine. At 7 days, transketolase activity was 81% of the value obtained with hepatocytes isolated from pair-fed control rats and gluconeogenesis from all the above substrates was normal, indicating an excess of transketolase activity present. By 14 days, however, gluconeogenesis was depressed (65-88% of control). At 28 days, both transketolase activity and gluconeogenesis from xylitol were markedly depressed (32 and 73% of control, respectively), but moreover, those substrates not utilizing thiamine-requiring enzymes for gluconeogenesis were equally or more greatly depressed in their gluconeogenic rates (42-70% of control). These data indicate (i) transketolase activity is present in amounts greater than that needed for normal hexose monophosphate shunt operation, and (ii) thiamine deficiency markedly depresses hepatic gluconeogenesis from a variety of substrates, indicating a generalized effect of the absence of this vitamin on metabolism, and more specifically, that the depression in gluconeogenesis from xylitol could be due to a decrease in the activity of non-thiamine-requiring enzymes. The need for caution in interpreting cause-and-effect relationships between enzymes and pathway flux is stressed.