Increase in cell lipid and cytoplasmic particles in mammalian cells cultured at reduced pH.

The hydrogen ion concentration of the medium has been shown to exert a regulatory effect on the lipid content of cultured mammalian cells. Reduction of the pH of the medium from 7.4 to 6.9 causes a significant increase in cell lipid, relative to cell protein, within 2-3 days. Triglycerides are increased twofold and account for 75% of the additional lipid. Polar lipids, on the other hand, remain nearly constant in concentration. Concurrent with the increase in lipid, particles with an average diameter of 1 micro appear in the cytoplasm. Because the density of these particles is low, ultracentrifugation of the cell homogenate separates the particles completely from the other subcellular structures. The amount of lipid in the particle fraction is approximately equal to the increase in total cell lipid. As shown by silicic acid column chromatography, the particle lipid contains about 75% triglycerides, 15% diglycerides plus an unknown substance, and smaller amounts of material in the monoglyceride and sterol ester-hydrocarbon fractions. The quantitative results indicate that the lipid accumulated at low pH is assembled into discrete cytoplasmic particles.