Labeled oxidation products from [1‐14C], [U‐14C] and [16‐14C]‐palmitate in hepatocytes and mitochondria

Abstract

When [1-¹⁴C], [U-¹⁴C], and [16-¹⁴C]palmitate were oxidized by isolated rat hepatocytes, there was a differential distribution of label as a percent of total oxidized products, such that¹⁴CO₂ from [1-¹⁴C]>[U-¹⁴C]>[16-¹⁴C]-palmitate and acid-soluble radioactivity from [16-¹⁴C]>[U-¹⁴C]>[1-¹⁴C]palmitate. The oxidation of [2,3-¹⁴C]succinate to¹⁴CO₂ by isolated hepatocytes was only 9.1% of that from [1,4-¹⁴C]succinate, demonstrating that the differences in distribution of labeled products are in part due to less¹⁴CO₂ production from label in the even carbon positions entering the citric acid cycle. Apparent total ketone body production from [16-¹⁴C]palmitate was markedly higher than [1-¹⁴C], and [U-¹⁴C]palmitate. In addition, the¹⁴C-acetone:¹⁴CO₂ ratio derived from decarboxylation of labeled acetoacetate from [1-¹⁴C]palmitate was less than 1 and positively correlated to the rate of fatty acid oxidation in hepatocytes. These findings indicate that the known preferential incorporation of the omega-C₂ unit of fatty acids into¹⁴C-ketone bodies also contributed to the differential distribution of labeled products and that this contribution was greatest at the lower rates of fatty acid oxidation. In isolated mitochondria, the distribution of label to¹⁴CO₂ and acid-soluble radioactivity from [1-¹⁴C], [U-¹⁴C] and [16-¹⁴C]palmitate was qualitatively similar to that seen with hepatocytes. The distribution of label from [1-¹⁴C]acetylcarnitine to¹⁴CO₂ and¹⁴C-ketone bodies by mitochondria was identical to that observed from [1-¹⁴C]palmitate, indicating that the higher rates of¹⁴CO₂ production from [1-¹⁴C]palmitate cannot be explained by a preferential oxidation in the citric acid cycle of either extramitochondrial acetyl-CoA (generated in peroxisomes) or the carboxyl terminal of the fatty acid. As shown by others in cell-free systems, we observed that the total oxidation of [16-¹⁴C]palmitate by hepatocytes and mitochondria was significantly less than [1-¹⁴C] and [U-¹⁴C]palmitate, suggesting either incomplete mitochondrial β-oxidation or incomplete degradation of peroxisomal oxidation products. The data indicate that this incomplete oxidation does not, however, contribute to the differential distribution of label to oxidized products.