Degradation and regurgitation of extracellular proteins by cultured mouse peritoneal macrophages and baby hamster kidney fibroblasts. Kinetic evidence that the transfer of proteins to lysosomes is not irreversible.

The uptake and degradation of bovine serum albumin (BSA), bovine liver catalase, and rabbit muscle enolase have been studied in cultured mouse peritoneal macrophages (MPM) and baby hamster kidney fibroblasts (BHK cells). Rates constant for the uptake of the three proteins by MPM were similar. In addition, BSA accumulation was independent of BSA concentration in the uptake medium and was not inhibited by a large excess of serum, suggesting that protein accumulation was by fluid phase pinocytosis. Following an overnight uptake, 20-30% of the accumulated protein was subsequently regurgitated into the medium in a trichloroacetic acid/phosphotungstic acid-precipitable form. This material co-migrated with the authentic protein during molecular sieve chromatography on Sephadex G-50. The rates of appearance of trichloroacetic acid/phosphotungstic acid-insoluble products were greater than expected for cell death and leakage. The observed first order rate constants, kobs, for the appearance of trichloroacetic acid/phosphotungstic acid-soluble and trichloroacetic acid/phosphotungstic acid-insoluble products in the culture medium were identical, indicating that both products were released in parallel from MPM and BHK cells. The kobs for intracellular BSA degradation and regurgitation were independent of the initial BSA concentration in the uptake medium, but were decreased about 35% when degradation was allowed to proceed in the presence of high concentrations of serum. Degradation was also inhibited by chloroquine and pepstatin. Inhibition of degradation was accompanied by an increase in the total amount of regurgitated protein appearing in the medium. Remarkably, however, these inhibitors also decreased kobs for regurgitation, thereby preserving the similarity in the observed rate constants for the appearance of trichloroacetic acid/phosphotungstic acid-soluble and trichloroacetic acid/phosphotungstic acid-insoluble products. These and other results were inconsistent with desorption of proteins from the surface of the culture dish or the surface of cells as the source of the trichloroacetic acid/phosphotungstic acid-insoluble label appearing in the medium.(ABSTRACT TRUNCATED AT 400 WORDS)