AbstractCultured heart muscle cells, but not HeLa cells, oxidize long‐chain fatty acids in medium containing dialyzed serum. Addition of chicken serum dialysate (or non‐dialized serum) stimulated palmitic acid oxidation by HeLa cells 10 to 20 fold. This serum activity was not eliminated by lipid extraction, ethanol or acid precipitation, alkaline phosphatase treatment, or autoclaving. About 80 was lost after any one of the following treatments: 6N HCl at 110°C for 16 hr, pepsin, Dowex cation exchange at pH 3, or 1N KOH at 100°C for 1 hr. Serum activity was separated into five or more peaks by gel filtration with Sephadex G‐10. Each of these peak fractions was further purified by HPLC using a cyanopropyl‐bonded resin. Carnitine, which is important for the transport of long‐chain fatty acids into mitochondria for oxidation, also stimulated the oxidation of palmitate. However, these serum factors are not known precursors to carnitine since its immediate precursor 4‐n‐trimethylaminobutyrate, did not stimulate palmitate oxidation. Total carnitine, including that in acylcarnitine compounds, was approxoimately 15 μM in the chicken sera to give ∼ 0.7 μM in the medium. Based on the fraction of total activity accountable by carnitine and fractional stability to acid, alkali, and pepsin, about 75 of the activity is from non‐carnitine compounds. Only one of the factors appears to be carnitine or an acylcarnitine derivative. Several lines of evidence suggest that the other factors
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