Effect of suramin on pinocytosis by resident rat peritoneal macrophages: an analysis using four different substrates

The effect of suramin on pinocytosis and intralysosomal proteolysis by resident rat peritoneal macrophages cultured in vitro has been studied. Suramin had little effect on the rate of pinocytic uptake of two non-adsorptive substrates [14C]sucrose and [3H]dextran, but unexpectedly enhanced uptake of a third, 125I-labelled polyvinylpyrrolidone (PVP). Since this enhanced uptake was completely abolished by NaF at a concentration known to inhibit pinocytosis, it clearly represented an increased internalization of substrate and not merely a superficial binding to the cell surface. It was concluded that suramin (i) does not affect the rate of formation of pinocytic vesicles but (ii) acts as a bivalent ligand, binding to both the macrophage surface and the 125I-labelled polyvinylpyrrolidone, thus converting a non-adsorptive into an adsorptive substrate. Suramin (500 micrograms/ml) decreased both the rate of uptake of formaldehyde-denatured 125I-labelled bovine serum albumin (BSA) (an adsorptive substrate) and the rate of its subsequent intracellular degradation. Thus, depending on the substrate chosen to measure pinocytosis, the same modifier may stimulate or inhibit uptake or be without effect.