Origin of hydrogen required for fatty acid synthesis in isolated rat adipocytes

Abstract

Isolated rat adipocytes were incubated in parallel either with [U- 14C]-, [1- 14C]-, or [6- 14C]glucose and unlabeled citrate or with [6- 14C]- or [1,5- 14C]citrate and unlabeled glucose. Glucose uptake, lactate and pyruvate release, and 14C-incorporation into CO 2, fatty acids, glyceride glycerol, and lactate were determined. The results obtained with variously labeled 14C-glucose revealed that the total amount of cytosolic NADH and NADPH was only in slight excess of the hydrogen equivalents required for the synthesis of fatty acids, glyceride glycerol, and lactate. The pentose phosphate pathway was found to provide only about 50–80% of the NADPH required for fatty acid synthesis, indicating, that part of NADPH is generated by transhydrogenation from cytosolic NADH. The isotopic yields of products from variously labeled 14C-citrate were in accord with an operating citrate malate pyruvate-cycle supplying the remaining NADPH required for fatty acid synthesis via malic enzyme. The ratios of the 14C-yields of lactate indicate a randomization of label at the dicarboxylic acid level which is considered as a consequence of an operating malate shuttle.