A radioisotopic technique for analysis of free fatty acid reesterification in human adipose tissue

Reesterification rates of free fatty acids (FFA) formed by intracellular triglyceride hydrolysis in small fragments of human adipose tissue were measured. Subcutaneous gluteal adipose tissue, obtained by needle biopsy, was incubated in a buffered albumin medium containing [3H]palmitate and [14C]glucose, each of high specific activity. In triglycerides (TG) and diglycerides (DG) synthesized by the tissue, [14C]glucose is incorporated exclusively into the glyceride-glycerol moiety, and 3H appears solely in the esterified fatty acids. Since rates of TG and DG synthesis can be determined from 14C accumulation rates in these molecules, the total amounts of FFA esterified can also be calculated. The difference between this estimate of total FFA esterification and the moles of [3H]palmitate esterified to these molecules represents the amount of unlabeled FFA from ongoing TG hydrolysis that was reesterified during the incubation. FFA recycling by the reesterification pathway is an important mechanism for the control of the quantity and proportions of FFA and glycerol leaving the human adipocyte. Fasting and beta-adrenergic stimulation reduce the fraction of endogenously released FFA that are reesterified from resting values of 30-40% to 8-21%, thereby increasing the molar ratio of FFA to glycerol leaving the adipocyte. The technique described can be employed to monitor sequential changes in this important metabolic cycle in humans under a wide range of nutritional and clinical circumstances.