1. The ability of fat-free homogenates of human adipose tissue to incorporate [1-14C]palmitate into neutral lipid and long-chain fatty acyl-CoA has been studied. Homogenates readily incorporated [1-14C]palmitate into neutral lipid. Addition of l-glycerol 3-phosphate was required before synthesis of neutral lipid occurred whereas the system contained sufficient endogenous fatty acid for esterification with l-glycerol 3-phosphate.
2. Glucose, glucose 6-phosphate and fructose 1,6-diphosphate could replace l-glycerol 3-phosphate if the system was supplemented with NADH. The order of efficiency of each substrate to support esterification of [1-14C]palmitate was: l-glycerol 3-phosphate, fructose, 1,6-diphosphate, glucose 6-phosphate and glucose.
3. Factors known to alter the activity of phosphofructokinase (E.C.18.104.22.168) also altered the rates of incorporation of [1-14C]palmitate into neutral lipid. Thus citrate (10 mm) inhibited lipogenesis in this system when glucose 6-phosphate was used as initial substrate, but not when fructose 1,6-diphosphate was used. 3′5′-(cyclic)-AMP reversed the inhibition of lipogenesis by citrate.
4. Preparations of lipomata (benign tumours of adipose tissue) were found to be less sensitive to inhibition of lipogenesis by citrate (7 mm) but inhibition was observed at higher concentrations of citrate (21 mm). 3′5′-(cyclic)-AMP had no effect on lipogenesis in fat-free homogenates of lipomata in the presence of citrate (7 mm).
5. AMP (1·2 mm) inhibited the synthesis of long-chain fatty acyl-CoA without altering the rate of synthesis of neutral lipid from l-glycerol 3-phosphate. When the concentration of AMP was increased to 4 mm a concurrent decrease in lipid synthesis was observed.