Flux of palmitate through the peroxisomal and mitochondrial beta-oxidation systems in isolated rat hepatocytes.

Peroxisomes catalyze the beta-oxidation of fatty acids but their quantitative role in fatty acid catabolism in the intact hepatocyte is not yet clarified. In the present study peroxisomal beta-oxidation of [1-14C]palmitate was quantitated in hepatocytes without the use of metabolic inhibitors. It was assumed that acetyl-CoA formed by peroxisomal beta-oxidation enters the cytosolic pool of acetyl-CoA, whereas that from mitochondrial beta-oxidation enters the mitochondrial pool. The labeling of the two acetyl-CoA pools was assessed by measuring the incorporation of radioactivity into cholesterol (from cytosolic acetyl-CoA) and CO2 (from mitochondrial acetyl-CoA). The system was calibrated with [1-14C]acetate and [1-14C]butyrate because butyrate undergoes beta-oxidation only in mitochondria, whereas acetate forms acetyl-CoA primarily in the cytosol. The labeling ratio, [( 14C]cholesterol X 100)/[( 14C]cholesterol + [14C]CO2), reflects the site of formation of acetyl-CoA. This ratio was 0.51 for butyrate, 1.39 for acetate and 0.79 for palmitate. The difference between palmitate and butyrate was statistically significant (P less than 0.02). This indicates that not all of the palmitate was oxidized in mitochondria. By linear interpolation it was estimated that approximately 32% of the [1-14C]palmitate oxidation began in peroxisomes.