Heterogeneity of binding site for adsorptive pinocytosis of simple proteins by rat yolk sacs.

1. When rat yolk sacs were incubated in serum-free medium 199, the 125I-labelled forms of both formaldehyde-treated bovine serum albumin and ribonuclease were captured far more rapidly than 125I-labelled polyvinylpyrrolidone. Extensive adsorption of these proteins to the plasma membrane is the main cause of this effect. 2. Quantitative analysis of the adsorptive-phase pinocytosis of these two proteins showed curved Hofstee plots, suggesting either 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. The values of Km were similar, ranging over 0.6-11.8 microM for 125I-labelled ribonuclease and over 0.31-4.7 microM for formaldehyde-treated 125I-labelled albumin. 3. Competitive uptake studies, in which tracer amounts of each of the 125I-labelled proteins were ingested from serum-free medium containing a higher concentration of one of a series of unlabelled proteins, revealed marked differences between the two radiolabelled proteins. These findings suggest that formaldehyde-denatured albumin is captured by binding to hydrophobic binding sites on the plasma membrane whereas ribonuclease is captured by binding to negatively charged sites.