Three cholesterol-esterifying enzymes purified recently to apparent homogeneity from rat liver cytosol (Hradec, et al: J Chromatogr B 681: 55-62, 1996) showed in vitro an absolute requirement for natural mixtures of phosphatidylcholines. These phospholipids may be replaced by the addition of minute quantities of serum from cancer patients but not by that of healthy individuals. Individual enzymes showed different sensitivities in this respect. They utilized only phosphatidylcholines containing saturated fatty acids as substrates but not those containing unsaturated fatty acids. As revealed by high-performance liquid chromatography, products of the esterification were esters with saturated C12-C18 fatty acids (including odd-numbered) in comparable proportions. If cancer serum was added as the only source of substrate, enzymes synthesized predominantly cholesteryl 14-methylhexadecanoate. The enzymatic activity and nature of reaction products thus depended on the availability of particular phosphatidylcholines present in the cancer serum but not (or in lower quantities) in the serum of healthy individuals. These results may be of significance for further studies on metabolic changes accompanying the growth of malignant tumours.

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