Inhibitory effect on replicative DNA polymerases, human cancer cell proliferation, and in vivo anti-tumor activity by glycolipids from spinach

We succeeded in purifying a major glycolipids fraction (i.e., Fraction-II) in the class of monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG) and sulfoquinovosyl diacylglycerol (SQDG) from spinach using hydrophobic column chromatography. Fraction-II inhibited the activities of replicative DNA polymerases (pols) such as alpha, delta and epsilon, and mitochondrial pol gamma with IC50 values of 43 - 79 mu g/ml, but had no influence on the activity of repair-related pols P and X. MGDG, DGDG, SQDG were purified from Fraction-II of spinach using silica gel column chromatography, and SQDG was the strongest inhibitor of mammalian pols in the three glycolipids. Therefore, SQDG and its related compounds were chemically synthesized, and the sulfate group and fatty acid moiety of the compound were suggested to be important for pol inhibition. These glycolipids showed no effect even on the activities of plant pols, prokaryotic pols and other DNA metabolic enzymes such as T4 polynucleotide kinase, T7 RNA polymerase and deoxyribonuclease I. Fraction-II also inhibited the proliferation of human cervix carcinoma (HeLa) cells with LD50 values of 57 mu g/ml, and could halt the cell cycle at the G1-phase, and subsequently induced severe apoptosis. In an in vivo anti-tumor assay on nude mice bearing solid tumors of HeLa cells, Fraction-II was shown to be a promising suppressor of solid tumors. Histopathological examination revealed that tumor necrosis with hemorrhage was significantly enhanced with Fraction-II in vivo. The spinach Fraction-II containing SQDG might be a potent anti-tumor compound, and may be a healthy food substance with antitumor activity.