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

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Abstract
Labelling of the phospholipids, fatty acids from the acetone-soluble lipid, and non-esterified cholesterol in slices of rat and guinea pig liver respiring in a suitably buffered Krebs-Ringer medium containing acetate-1-C14 was studied. The time course of the reactions and effects of the concentration of K ion and pH of the incubation medium were defined. For phospholipid and fatty acids of 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 O2 of the gas phase was replaced with N2, labeling of all 3 lipid fractions was abolished. Addition of glucose to the incubation medium slightly increased the labelling of phospholipids and fatty acids of acetone-soluble lipid, but had no consistent effect on labelling of non-esterified cholesterol. Purification of cholesterol by the method of bromination and debromination caused only a slight change in specific activity, indicating that 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 O2 consumption. Malonate inhibited the incorporation of acetate-1-C14 into cholesterol, but did not affect labelling of phospholipids. When acetate-1-C14 was replaced with other C14-labelled precursors, good labelling of phospholipids was observed with glycine-2-C14, glycerol-l-Cl4, and fructose-Cl4, but not with fromate-Cl4, lactate-1-C14, or glucose-C14. Cholesterol was not significantly labelled from any of the precursors other than acetate-l-C14.