ACTIVITY OF CHOLESTEROL-ESTERIFYING ENZYMES FROM RAT-LIVER CYTOSOL ISAFFECTED BY MALIGNANT GROWTH

Three cholesterol-esterifying enzymes purified recently to apparent ho mogeneity from rat liver cytosol (Hradec, et al: J Chromatogr B 681: 5 5-62, 1996) showed in vitro an absolute requirement for natural mixtur es of phosphatidylcholines. These phospholipids may be replaced by the addition of minute quantities of serum from cancer patients but not b y that of healthy individuals. Individual enzymes showed different sen sitivities in this respect. They utilized only phosphatidylcholines co ntaining saturated fatty acids as substrates but not those containing unsaturated fatty acids. As revealed by high-performance liquid chroma tography, products of the esterification were esters with saturated C1 2-C18 fatty acids (including odd-numbered) in comparable proportions. If cancer serum was added as the only source of substrate, enzymes syn thesized predominantly cholesteryl 14-methylhexadecanoate. The enzymat ic activity and nature of reaction products thus depended on the avail ability 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.