Cultured heart muscle cells, but not HeLa cells, oxidize long-chain tatty 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 HCI at llOΒ°C for 16 hr, pepsin, Dowex cation exchange at pH 3, or I N KOH at 100Β°C for 1 hr. Serum activity was separated into tive or more peaks by gel tiltration with Sephadex G-10. Each of these peak tractions was iurther purified by HPLC using a cyanopropyl-bonded resin. Carnitine, which is important ior the transport o i 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 4n-trimethylaminobutyrate, did not stimulate palmitate oxidation. Total carnitine, including that in acylcarnitine compounds, was approxoimately 15 pM in the chicken sera to give -0.7 pM in the medium. Based on the traction ot total activity accountable by carnitine and tractional stability to acid, alkali, and pepsin, about 75% of the activity I S trom non-carnitine compounds. Only one of the tactors appears to be carnitine or an acylcarnitine derivative. Several lines of evidence suggest that the other factors are peptide compounds.
The oxidation of fatty acids is a n important source of energy for animals and plants; in some tissues, such as cardiac muscle, it may be the major source (Carlsten et al., 1961;Neely et al., 1972). Fatty acids are formed from triacylglycerols in adipose tissue, and while tightly bound to albumin, are transported to tissues via the blood. They are activated with Coenzyme A (CoASH), plus adenosine-5'-triphosphate (ATP) to give acyl-CoA thioesters. At least three specific enzymes participate in this reaction which are specific for short-chain (2 to 41, medium-chain (4 to 12), or long-chain (12 to > 22) carbon fatty acids. The fatty acids are oxidized in mitochondria