Macromolecular interactions and connective-tissue metabolism.

of aortic cells, the pinocytosis proceeded with high efficiency. The uptake remained linear with time for at most 24h and then reached a plateau, probably owing to the competition for uptake by newly secreted unlabelled proteoglycans. A considerable amount of added proteoglycans, however, became associated with the cell surface (pericellular pool of glycosaminoglycans) and are not removed by repeated washing with Hanks solution. The association of 35S-labelled proteoglycans with the pericellular pool is time-dependent and requires about 4h until the steady state is reached, but only a small part of this pool represents material required for immediate pinocytosis. The incorporation of proteoglycans into the pericellular pool cannot be explained by adsorption, since this process is in part temperature-dependent. From the kinetics of adsorption and uptake processes it has to be concluded that binding for integration into the pericellular pool and for pinocytotic uptake appear as separate processes. This is suggested by the observation that the proteoglycan concentration that is required for a half-maximal saturation of the pericellular pool is lower than that for the half-maximal rate of pinocytosis. Adsorptive pinocytosis exhibits specificity, the individual proteoglycans being internalized at different rates. The highest rate of uptake was measured for a dermatan sulphate-rich proteoglycan. No competition of uptake between dermatan sulphate-rich and heparan sulphate-rich proteoglycans was observed. Competition experiments suggest that the individual proteoglycans are recognized by different receptors on the cell surface. Structural integrity is one of the prerequisites for optimal pinocytosis. Proteolytic digestion of the protein core or elimination of the polysaccharide chain by alkali results in marked decrease in uptake by arterial fibroblasts.