11). Under basal metabolic conditions, portions of the cytoplasm, which contain the entire cohort of cellular proteins, are segregated within a membrane-bound compartment and are then fused to a primary nascent lysosome and their contents digested. This process was called microautophagy. Under more extreme conditions, starvation for example, mitochondria, endoplasmic reticulum membranes, glycogen bodies, and other cytoplasmic entities can also be engulfed by a process called macroautophagy (see, for Inhibitors,research,lifescience,medical example, Ashford et al.12; the different modes of action of the lysosome in digesting extra- and intracellular proteins are shown in Figure 2). However, over a period

of more than two decades, between the mid-1950s and the late 1970s, it has become gradually more and more difficult to explain several aspects of intracellular protein degradation based on the known mechanisms of lysosomal activity: accumulating lines of independent experimental evidence Inhibitors,research,lifescience,medical indicated that the degradation of at least certain classes of cellular proteins must be non-lysosomal. Yet, in the absence of any “http://www.selleckchem.com/products/ink128.html alternative,” researchers came up with different explanations, some more substantiated and others less, to Inhibitors,research,lifescience,medical defend the “lysosomal” hypothesis. First was the gradual discovery, from different laboratories, that different proteins vary in their stabilities and their half-life times can span three orders of magnitude, from a few minutes to many

days. Thus, the t1/2 of ornithine decarboxylase Inhibitors,research,lifescience,medical (ODC) is ~10 min, while that of glucose-6-phosphate dehydrogenase (G6PD) is 15 hours (for review articles, see, for example, Schimke et al.13 and Goldberg et al.14). Also, rates of degradation of many proteins were shown to change with changing physiological conditions, such as availability of nutrients or hormones. It was conceptually

difficult to reconcile the findings of Inhibitors,research,lifescience,medical distinct and changing half-lives of different proteins with the mechanism of action of the lysosome, where the microautophagic vesicle contains the entire cohort of cellular (cytosolic) proteins that are therefore expected to degrade at the same rate. Similarly, changing pathophysiological conditions, such as starvation or resupplementation of nutrients, were Etomidate expected to affect the stability of all cellular proteins to the same extent. Clearly, this was not the case. Another source of concern about the lysosome as the organelle in which intracellular proteins are degraded was the finding that specific and general inhibitors of lysosomal proteases have different effects on different populations of proteins, making it clear that distinct classes of proteins are targeted by different proteolytic machineries. Thus, the degradation of endocytosed/pinocytosed extracellular proteins was significantly inhibited, a partial effect was observed on the degradation of long-lived cellular proteins, and almost no effect was detected on the degradation of short-lived and abnormal/mutated proteins.