Conversion of carbohydrate to fat in adipose tissue: an energy-yielding and, therefore, self-limiting process.

A theoretical analysis of the energy metabolism associated with the conversion of glucose to fat is presented. In tissues where the pentose cycle furnishes some of the NADPH required for fatty acid synthesis, this conversion is an ATP-yielding process. In rat adipose tissue the maximal rate of glucose conversion to fat can be quantatively predicted on the basis of the tissue's ability to use the ATP which is generated in excess during this conversion. The energy-generating nature of this process provides the means for a type of regulation which depends on metabolic state and which, during fasting, contributes to the sparing of carbohydrate. Impairment of lipogenesis in the fasting state is attributed to a decrease in the activity of the malate cycle and to the presence of free fatty acids. However, rather than by inhibiting specific enzymes, it is by virtue of their quality as substrates for energy production that free fatty acids and their CoA derivatives appear to inhibit de novo lipogenesis. The regulatory phenomena discussed here may explain the failure of the attempts made to identify the rate-limiting step for de novo lipogenesis in adipose tissue.