PHOSPHOLIPID METABOLISM IN LIVER SLICES: LABELLING OF PHOSPHOLIPIDS WITH ACETATE-1-C14

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Abstract
A study has been made of the labelling of the phospholipids, the fatty acids from the acetone-soluble lipid, and the non-esterified cholesterol in slices of rat and guinea pig liver respiring in a suitably buffered Krebs–Ringer medium containing acetate-1-C14. The time course of the reactions and the effects of the concentration of potassium ion and the pH of the incubating medium have been defined. For phospholipid and fatty acids of the acetone-soluble lipid, the optimum pH was in the range 6.8–7.4, whereas for cholesterol there was a much sharper optimum at pH 6.6–6.8. When the oxygen of the gas phase was replaced with nitrogen, the labelling of all three lipid fractions was abolished. The addition of glucose to the incubating medium slightly increased the labelling of the phospholipids and the fatty acids of the acetone-soluble lipid, but had no consistent effect on the labelling of the non-esterified cholesterol. Purification of the cholesterol by the method of bromination and debromination caused only a slight change in specific activity, indicating that the cholesterol was not contaminated with large amounts of companion substances with specific activities greatly different from that of the cholesterol itself. The addition of cyanide, fluoride, iodoacetate, or 2,4-dinitrophenol to the incubating medium caused a great decrease in the labelling of all fractions studied. With the exception of 2,4-dinitrophenol, the inhibitors were used in concentrations that inhibit the oxygen consumption. Malonate inhibited the incorporation of acetate-1-C14 into cholesterol, but did not affect the labelling of the phospholipids. When the acetate-1-C14 was replaced with other C14-labelled precursors, good labelling of phospholipids was observed with glycine-2-C14, glycerol-1-C14, and fructose-C11, but not with formate-C14, lactate-1-C14, or glucose-C14. The cholesterol was not significantly labelled from any of the precursors other than acetate-1-C14.

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