Heterogeneity of Binding Sites for Adsorptive Pinocytosis of Simple Proteins by Rat Yolk Sacs

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Abstract
When rat yolk sacs were incubated in serum-free medium 199, the 125I-labeled forms of both formaldehyde-treated bovine serum albumin and ribonuclease were captured far more rapidly than 125I-labeled polyvinylpyrrolidone. Extensive adsorption of these proteins to the plasma membrane is the main cause of this effect. Quantitative analysis of the adsorptive-phase pinocytosis of these 2 proteins showed curved Hofstee plots, suggesting the presence of multiple classes of binding site on the surface of pinocytically active yolk-sac cells or a single class of binding site that exhibits negative cooperativity in the binding of these proteins. Values of Km were similar, ranging over 0.6-11.8 .mu.M for 125I-labeled ribonuclease and over 0.31-4.7 .mu.M for formaldehyde-treated, 125I-labeled albumin. Competitive uptake studies, in which tracer amounts of each of the 125I-labeled proteins were ingested from serum-free medium containing a higher concentration of one of a series of unlabeled proteins, revealed marked differences between the 2 radiolabeled proteins. Apparently formaldehyde-denatured albumin is captured by binding to hydrophobic binding sites on the plasma membrane; ribonuclease is captured by binding to negatively charged sites.