We have studied the effect of temperature on uptake and degradation of molecules entering mouse peritoneal macrophages by fluid-phase, adsorptive and receptor-mediated pinocytosis, and on degradation of their intracellular proteins. Uptake of [3H]sucrose and uptake and degradation of formaldehyde-treated 125I-labelled human serum albumin and 125I-labelled mannose-bovine serum albumin continued, but were progressively slowed as the temperature decreased from 37 degrees C to 20 degrees C. The uptake and degradation were completely abolished at approximately 15 degrees C. Arrhenius plots for adsorptive and fluid uptake were unilinear, whereas that for receptor-mediated endocytosis showed an inflection point at approximately 20 degrees C. The results did not indicate any distinction between adsorptive and fluid pinocytosis. An ‘intracellular turnover time’ calculated for mannose-bovine serum albumin taken up by the specific route is 19–24 min and this time calculated for human serum albumin is, in contrast, 99 min. Studies of the kinetics of degradation of both endocytosed and endogenous proteins showed similarity in the temperature cut-off of degradation of endocytosed and endogenous long-half-life proteins (congruent to 15 degrees C) and continuance of endogenous short-half-life degradation at much lower temperatures.

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