The metabolism of dl-(1,6-14C)lipoic acid in the rat

dl-[1,6-¹⁴C]Lipoic acid was synthesized and administered to rats or incubated in vitro with rat liver systems. The urinary excretion of radioactivity after labeled lipoate was administered intraperitoneally at a level of 0.5 mg/100 g body weight was maximal at 3–6 hr, with 60% of the injected radioactivity recovered within 24 hr. Respiratory ¹⁴CO₂ from the same animals is maximal at 3 hr, after which it falls off markedly. Approximately 30% of the injected radioactivity was recovered as ¹⁴CO₂ within 24 hr. The excretion of radioactivity after lipoate was administered by stomach tube was similar to that after intraperitoneal injection. Localization of radioactivity in the body was greatest in liver, intestinal contents, and muscle in all cases. Ionexchange and paper chromatographies of 24-hr pooled urine revealed several watersoluble radioactive metabolites. Incubation of [¹⁴C]lipoate with homogenates or mitochondrial preparations in vitro resulted in the production of ¹⁴CO₂, which was decreased by incubation with unlabeled fatty acids and unaffected by the addition of carnitine or (+)-decanoylcarnitine. The rat, like certain bacteria, metabolizes lipoate via β-oxidation of the valeric acid side chain and by other metabolic reactions on the dithiolane ring, which render the molecule more water soluble.     

Incorporation of 1-(14)C linoleic acid in rat liver nuclei and chromatin fractions

The incorporation of 1-(14)C linoleic acid in several chromatin fractions of rat liver nuclei was investigated using two different procedures: (1) rat liver nuclei were incubated with ATP, CoASH, Mg(++) and 1-(14)C linoleic acid. After 40 min at 37 degrees C the chromatin obtained by sonication of nuclei suspended in 0.25 M sucrose was fractionated by differential sedimentation; (2) chromatin fractions obtained by differential sedimentation were incubated separately with ATP, CoASH, Mg(++) and 1-(14)C linoleic acid 40 min at 37 degrees C in order to characterize the fatty acid incorporation in isolated chromatin. A comparative study of the incorporation of 1-(14)C linoleic acid in microsomes and nuclei isolated from rat liver is also presented for the purpose of comparison. Linoleic acid was incorporated into nuclear lipids as well as in chromatin fractions. The fatty acid incorporation was stimulated considerably in the acylation system when compared to control, it appears to be highly dependent on the state of condensation of chromatin, being barely detectable in the lowest density fraction. The major proportion of 1-(14)C linoleic acid was found in phospholipids and in a lesser proportion it remained esterified to triglycerides and cholesteryl esters. The distribution of radioactivity in different classes of phospholipids present in microsomes and nuclei isolated from rat liver, showed a similar profile of distribution. The major proportion of radioactivity, approximately 50% was found in phosphatidylcholine and in a lesser proportion in sphingomyelin, phosphatidylserine and phosphatidylethanolamine. When chromatin fractions were incubated separately, it was observed that the major proportion of 1-(14)C linoleic acid in phospholipids was found in heavy chromatin fractions whereas low density chromatin fraction only incorporated in a lesser proportion.     

The metabolic fate of (2-14C)folic acid and a mixture of (2-14C)- and (3'',5'',9-3h)-folic acid in the rat.

The metabolism of [2-14C]folic acid over 13 days and a mixture of [2-14C]- and [3',5',9-3h]-folic acid in rats over a 6-day period is described. Both 14C and 3H are excreted in urine over the 6-day period, but 3H and 14C are only detectable in faeces for 2 days. A breakdown product of folic acid labelled with 3H only was found in some urine samples, but no metabolite corresponding to the part of the molecule containing 14C was detected. These experiments show that in the whole animal a substantial portion of orally administered folic acid undergoes scission shortly after administration [Blair Biochem. J. (1957) 68, 385-387] and that the retained folates are a shortage form for folate monoglutamates.     

Conversion of (U-14C)-glycerol, (2-3H)-glycerol and (1-14C)-palmitate into circulating lipoproteins in the rat.

The in vivo formation of labelled very low density lipoproteins (VLDL) from (U-14C)-glycerol, (2-3H)-glycerol and (1-14C)-palmitate was studied in fed female rats. The rate of disappearance of radioactivity from plasma after the i.v. injection with these tracers was similar for (U-14C)-glycerol and (1-14C)-palmitate. With (2-3H)-glycerol, plasma radioactivity at 10 min was lower than with the other substrates although it did not change thereafter. A certain proportion of radioactivity administered as glycerol appeared in plasma lipids, mainly in the VLDL glyceride glycerol fraction, although when (U-14C)-glycerol was the substrate, a considerable portion also appeared in the esterified fatty acids of these lipoproteins. When using (1-14C)-palmitate, practically all the circulating labelled esterified fatty acids appeared in the VLDL fraction, while the labelled free fatty acids appeared in lipoprotein of higher density, presumable free fatty acid-albumin complexes. This data is discussed in terms of the role of the liver in the rapid, continuous cycling of these substrates to yield VLDL-glycerides for their extrahepatic utilization.