Metabolism of Free Fatty Acids in Isolated Liver Cells

The structural and metabolic integrity of isolated rat liver cells was verified by the high percentage of trypan blue exclusion, a low degree of lactate dehydrogenase release into the medium, a constant rate of gluconeogenesis from L( +)lactate, and increased oxygen consumption following the addition of 2,4-dinitrophenol. Pahnitic acid, incubated in an albumin-bound form with isolated liver cells, was esterified to form phospholipids, triglycerides, diglycerides, and cholesterol esters and was oxidized to CO2 and ketone bodies. In liver cells from fed rats, the major portion of pahnitate was esterified, an intermediate quantity was oxidized to ketone bodies, and a smaller amount was oxidized to COz. The partition of pahnitic acid between esterification and ketogenesis was inversed by fasting, whereas oxidation to CO2 and the total rate of pahnitate utilization were unaltered. Greater esterification of [‘“Clpalmitate in cells from fed rats was not a result of carbon recycling via chain elongation or de novo synthesis. Liver cells from fasted rats derived more energy from fatty acid oxidation than cells from fed rats. The results indicate that citric acid cycle flux and endogenous lipolysis were unaffected by fasting. These observations signify that altered partition of free fatty acids between the pathways of oxidation and esterification in the liver is a major causative factor in the increased ketogenesis in the fasting state. An increase in [1-14C]pahnitate concentration augmented pahnitate uptake, ketogenesis, and esterification, whereas W02 production was only slightly affected. The estimated citric acid cycle flux and the acetoacetate to P-hydroxybutyrate ratio were diminished. Increased ketogenesis in response to sequential elevation of the pahnitate concentration could not be accounted for by diminished citric acid cycle flux and therefore resulted from increased /3 oxidation. Ketone body specific activity approached a constant value at ij of 3 to 4. Results indicate intracellular mixing of free fatty acids derived from endogenous lipolysis and from the medium, prior to /3 oxidation. Phospholipid was the predominant esterification product at low concentrations of