title: The degradation of endogenous and exogenous proteins in cultured smooth muscle cells.
abstract: The pathways of degradation followed by endogenous proteins in cultured smooth muscle cells were compared with the well-characterized lysosomal pathway involved in the degradation of apolipoprotein B of endocytosed LDL. Under conditions in which lysosomal activity towards 125I-labeled LDL was almost completely inhibited by chloroquine and/or ammonium chloride, the degradation of short-lived and abnormal proteins, assessed by the release of [3H]phenylalanine, was reduced by only 10-17%. The basal rate of degradation of long-lived proteins was reduced by about 30% by the same inhibitors while the accelerated proteolysis found under nutrient-poor conditions could be completely accounted for by the lysosomal system as defined by these lysosomotrophic agents. Temperature studies indicated differences between the mechanisms involved in the degradation of long-lived proteins (Ea = 18 kcal/mol) and short-lived proteins (Ea = 10 kcal/mol). Arrhenius plots for the degradation of endogenous proteins showed no transitions between 15 and 37 degrees C in contrast to the breakdown of LDL which ceased below 20 degrees C. The results indicate that the degradation of rapid-turnover proteins is largely extralysosomal and that a significant breakdown of long-lived proteins occurs also outside lysosomes.
mesh_terms: D000818:Animals; D001013:Aorta, Thoracic / Q000378:metabolism; D002417:Cattle; D002460:Cell Line; D002738:Chloroquine / Q000494:pharmacology; D002918:Chymotrypsin / Q000037:antagonists & inhibitors; D004622:Embryo, Mammalian; D006801:Humans; D007700:Kinetics; D007976:Leupeptins / Q000494:pharmacology; D008077:Lipoproteins, LDL / Q000378:metabolism; D008247:Lysosomes / Q000378:metabolism; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism*; D009842:Oligopeptides / Q000494:pharmacology; D011506:Proteins / Q000378:metabolism*
chemical_list: D007976:Leupeptins; D008077:Lipoproteins, LDL; D009842:Oligopeptides; D011506:Proteins; D002738:Chloroquine; C002101:chymostatin; D002918:Chymotrypsin; C032854:leupeptin