Inhibition of thymidine incorporation into the DNA of normal and neoplastic cells by a factor from bovine maternal placenta: interaction of the inhibitor with cell membranes.

An inhibitor was isolated from the maternal part of bovine placentas which inhibits the incorporation of [3H]thymidine into the DNA of a variety of tumor cells to a significantly higher degree as compared to normal cells. This protein-type component was labeled by reaction with N-succinimidyl[2,3-3H]propionate, and interactions with receptors on cell membranes were investigated. Results indicated that receptors on tumor cell surfaces have higher binding capacities versus the inhibitor than those of normal cells. Moreover, an additional type of receptor was detected on tumor membranes. Obviously one of the reasons for the higher inhibitory capacity of the factor in tumor cells is the better internalization of this component.