Low temperature selectively inhibits fusion between pinocytic vesicles and lysosomes during heterophagy of 125I-asialofetuin by the perfused rat liver.

Galactose-terminating ligands are specifically recognized by mammalian hepatocytes, internalized, and degraded within lysosomes. We have studied the effects of temperature on this process using 125I-asialofetuin and the perfused rat liver. Uptake and catabolism of 125I-asialofetuin continued but were progressively slowed as the temperature decreased from 35 degrees C to 20 degrees C. At 20 degrees C, lysosomal degradation completely stopped. Results of subcellular fractionation experiments and in situ electron microscopic autoradiography revealed that fusion between pinocytic vesicles and lysosomes did not occur below 20 degrees C, despite the continued endocytosis of 125I-asialofetuin and movement of pinocytic vesicles into the lysosome-Golgi region of the cell. Below 10 degrees C, endocytosis essentially stopped. Inhibition of the fusion step was reversed upon rewarming the liver to physiological temperature. By examining the reversal process the half-time for fusion between pinocytic vesicles and lysosomes was determined to be 7 min. This is the slowest metabolic event involved in the catabolism of asialofetuin by the liver both in the perfused organ and in vivo.