| Abstrakt: |
The inhibitions of protein synthesis initiation in heme-deficient reticulocyte lysates and in GSSG-treated hemin-supplemented lysates are both characterized by the activation of heme-regulated eIF-2 alpha kinase, which phosphorylates the alpha-subunit of eukaryotic initiation factor (eIF-2). In both inhibitions, the accumulation of eIF phosphorylated in alpha-subunit (eIF-2(alpha P)) leads to the sequestration of reversing factor (RF) in a phosphorylated 15 S complex, RF.eIF-2(alpha P), in which RF is nonfunctional. A sensitive assay for the detection of endogenous RF activity in protein-synthesizing lysates indicates that, in GSSG-inhibited (1 mM GSSG) lysates, RF is more profoundly inhibited than in heme-deficient lysates. RF inactivation in GSSG-induced inhibition appears to be due to two separate but additive effects: (i) the formation of the phosphorylated 15 S RF complex, RF.eIF-2(alpha P), and (ii) the formation of disulfide complexes which inhibit RF activity. Both inhibitory effects are overcome by catalytic levels of exogenous RF which permits the resumption of protein synthesis. RF activity and protein synthesis in GSSG-inhibited lysates are efficiently restored by the delayed addition of glucose-6-P or 2-deoxyglucose-6-P (1 mM). The rescue of protein synthesis by hexose phosphate (1 mM) is proportional to the extent of RF recovery and is due in part to NADPH generation; even at levels of hexose phosphate (50 microM) too low to support protein synthesis, partial restoration of RF activity occurs due to increased NADPH/NADP+ ratios. The ability of dithiothreitol (1 mM) to restore RF activity in GSSG-treated but not heme-deficient lysates also provides evidence for a reducing mechanism which functions at the level of RF. The results suggest that NADPH plays a role in the maintenance of sulfhydryl groups essential for RF activity. |
