Differential labeling of triglycerides and polar lipids of cultured mammalian cells by albumin-bound [1-¹⁴C] fatty acids of serum

Summary

Rabbit liver cells were cultured in medium containing serum whose albumin-bound fatty acids were labeled with [1-¹⁴C] palmitic or oleic acid of determined specific activity. After 7 to 500 fold increases in cell mass, the cell lipid was extracted and fractionated by silicic acid column chromatography. The triglyceride and polar lipid fractions were saponified and their constituent fatty acids, in the form of methyl esters, were separated and isolated by gas chromatography and their specific activities determined. Based on their¹⁴C content, approximately three-quarters of the palmitic and oleic acids of the accumulated triglycerides, which constituted half of the cell lipid, were derived from their counterparts in the albumin-bound fatty acids of the medium. In the case of the structural polar lipids, approximately only one-half of the palmitic and oleic acids were derived from their albumin-bound counterparts. Since the presence of serum in the medium completely represses thede novo synthesis of fatty acids in cultured mammalian cells, it is concluded that an appreciable portion of the polar lipid fraction is derived from the complex lipids of the serum lipoproteins, or their partial hydrolysis products. Based on these considerations, a function of serum lipoproteins is to act as precursors of a portion of the cell's structural lipids, or constituent parts thereof.

Within the cell, [1-¹⁴C] palmitic acid was converted to radioactive stearic, oleic, and palmitoleic acids. [1-¹⁴C] oleic acid, however, was neither reduced nor converted in detectable amounts to polyenoic fatty acids. Comparison of the rabbit serum albumin-bound fatty acids with the fatty acids of the cell's complex lipids showed that the latter contained lower concentrations of C16:0 and higher concentrations of C18:0 and C20:4 fatty acids than did the albumin. Also, within the cell, C16:0 was higher in the accumulated triglycerides whereas C18:0 and C20:4 were higher in the polar lipids. Concentrations of C18:1 and C18:2 did not differ greatly in the two fractions, but the small amount of C18:3 was confined almost entirely to triglycerides.