In this chapter we have attempted to provide some insight into a mechanism(s) of protein degradation in liver which we believe is implicated in the physiological regulation of free amino acid pools. The association we have observed between regulatory effects of insulin and amino acids in the perfused rat liver and physical alterations of the lysosomal system in similarly treated livers suggests that the final stage of the process is localized within the lysosomal-vacuolar system. The present studies show that perfusion of the isolated rat liver is in itself a potent stimulus to general proteolysis, an effect which is associated with increases in lysosomal size, density and sensitivity to osmotic shock. Since the osmotic and density alterations can be prevented or reversed by the provision of insulin or amino acid mixtures, we believe these agents normally act to restrain some inherent proteolytic process. Although the attendant lysosomal alterations are similar in many respects to those induced by glucagon, they differ sharply from the latter in having no sequestered organelles or evident membrane structures and the fact that they rise in the absence of any increase in the tissue level of cyclic AMP. Aside from the obvious problem of evaluating the quantitative importance of the lysosomal system in hepatic proteolysis, three interesting problems remain to be solved: (1) the nature of the proteins involved, (2) their mode of acquisition by the lysosome, and (3) the relationship of this inherent proteolytic process to glucagon or cyclic AMP-induced cellular autophagy. With regard to the former it seems reasonably clear from our studies and those of others that protein substrate does not gain access to lysosomal proteases by the direct penetration of lysosomal membranes, but more likely by fusion of a preexisting lysosome with some, as yet undefined, membrane-bound element. Whether such an element is formed in a manner analogous to that of the precursor vacuoles in autophagy (Ericsson 1969) or by some other process is not known. Closely related to this is the question of the nature of the substrate and the factors determining its uptake by the lysosome. Space does not permit us to review evidence from a number of laboratories which suggests that the turnover of intracellular protein may be a function of size (Dehlinger and Schimke 1971; Dice et al. 1973), charge (Dice and Goldberg 1975), or conformational stability (Li and Knox 1972; Ballard et al. 1974). We have no idea at the moment whether the mechanism we have outlined will eventually be shown to encompass a wide range of turnover of intracellular proteins or, alternatively, whether it will be limited to a certain class of proteins that turnover as a unit. While there is undoubtedly far more specificity in the regulation of intracellular proteins than our present data indicate, we do not view this as an incompatibility, but rather as another aspect of a complex problem we are only beginning to understand.
|Original language||English (US)|
|Number of pages||28|
|Journal||Frontiers of biology|
|State||Published - Jan 1 1976|
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