| Abstract: | The degradation of nine well-defined proteins was studied in cultured mouse peritoneal macrophages following their uptake by fluid phase pinocytosis. After uptake, approximately one-third of the radioactivity was released into the medium in the form of trichloroacetic acid/phosphotungstic acid-insoluble material. When the time courses for the appearance of trichloroacetic acid/phosphotungstic acid-soluble and -insoluble radioactivities were independently analyzed, identical observed rate constants (kobs) were obtained. This is in agreement with an earlier claim that regurgitated protein and low molecular weight products arise from a common intracellular pool of radiolabeled substrates, presumably within lysosomes, and that the traffic of substrates between the plasma membrane and the lysosome is probably bidirectional (Buktenica, S., Olenick, S. J., Salgia, R., and Frankfater, A. (1987) J. Biol. Chem. 262, 9469-9476). When intrinsic degradation rate constants (kd) were calculated, these were found to vary inversely with protein subunit molecular weights, from 0.0347 h-1 for horse heart cytochrome c to 0.0104 h-1 for rabbit muscle phosphorylase b. The proportion of peptide bonds in a protein which are initially available to the action of lysosomal proteases should be proportional to the fraction of the total potential surface of a protein which remains accessible to solvent after polypeptide folding (AS/AT). In agreement, lysosomal degradation rates were observed to correlate well with known or estimated values of AS/AT, and thermal denaturation, which may expose previously buried amino acid residues, increased the rate of degradation of bovine serum albumin. |
