Adsorptive pinocytosis of acid hydrolases by fibroblasts depends on phosphomannosyl recognition markers on the enzymes and high-affinity pinocytosis receptors on the cell surface. In this study, ß-glucuronidase binding to the cell surface of attached fibroblasts was found to be saturable and inhibitable by mannose-6-phosphate (Man-6-P). Dissociation of cell-bound ß-glucuronidase occurred very slowly at neutral pH, but was greatly accelerated by lowering the pH below 6.0, or by exposure to Man-6-P. Comparison of the maximal cell surface binding and the observed rate of enzyme pinocytosis suggests that the pinocytosis receptors are replaced or reused about every 5 min . Enzyme pinocytosis was not affected by inhibition of new protein synthesis for several hours, suggesting a large pool of internal receptors and/or reuse of internalized receptors .Chloroquine treatment ofnormal human fibroblasts had three effects : (a) greatly enhanced secretion of newly synthesized acid hydrolases bearing the recognition marker for uptake, (b) depletion of enzyme-binding sites from the cell surface, and (c) inhibition of pinocytosis of exogenous enzyme . Only the third effect was seen in I-cell disease fibroblasts, which were also less sensitive than control cells to this effect .These observations are consistent with a model for transport of acid hydrolases that proposes that delivery of newly synthesized acid hydrolases to lysosomes requires the phosphomannosyl recognition marker on the enzymes, and intracellular receptors that segregate receptor-bound enzymes into vesicles for transport to lysosomes . This model explains how chloroquine, which raises intralysosomal pH, can disrupt both the intracellular pathway for newly synthesized acid hydrolases, and the one for uptake of exogenous enzyme by cell surface pinocytosis receptors .Adsorptive pinocytosis of lysosomal hydrolases was initially recognized as the uptake of "corrective factors" by enzyme-deficient fibroblasts (for review, see reference 14) . These corrective factors J . CELL BIOLOGY C The Rockefeller University Press -0021-9525/80/06/0839/14 $1.00 Volume 85 June 1980 839-852 proved to be acid hydrolases that were secreted into the culture medium by fibroblasts and were also present in body fluids and tissue extracts . The selectivity and saturability of the uptake system 839 on

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