Protein turnover in subcellular fractions of brain from the ethanol-fed rat

Acute exposure to the nutritional toxin alcohol reduces brain protein synthesis in vivo

Few studies have measured brain protein synthesis in vivo using reliable methods that consider the precursor pool, and there is a paucity of data on the regional sensitivity of this organ to nutritional or toxic substances. We hypothesized that different areas of the brain will exhibit variations in protein synthesis rates, which might also be expected to show different sensitivities to the nutritional toxin, ethanol. To test this, we dosed male Wistar rats with ethanol (75 mmol/kg body weight) and measured rates of protein synthesis (ie, the fractional rate of protein synthesis, defined as the percentage of the protein pool renewed each day; k(s), %/d) in different brain regions 2.5 hours later with the flooding dose method using L-[4-(3)H] phenylalanine. In the event that some regions were refractory to the deleterious effects of ethanol, we also predosed rats with cyanamide, an aldehyde dehydrogenase inhibitor (ie, cyanamide + ethanol), to increase endogenous acetaldehyde, a potent neurotoxic agent. The results indicated the mean fractional rates of protein synthesis in the cortex was 21.1%/d, which was significantly lower than either brain stem (30.2%/d, P <.025), cerebellum (30.1%/d, P <.01), or midbrain (29.8%, P <.025). Ethanol significantly decreased protein synthesis in the cortex (21%, P < 0.01), cerebellum (19%, P <.025), brain stem (44%, P <.025), but not in the midbrain (not significant [NS]). However, significant reductions in protein synthesis in the midbrain occurred in cyanamide + ethanol-dosed rats (60%, P <.0001). Cyanamide + ethanol treatment also reduced k(s) in the brain stem (66%, P <.001), cortex (59%, P <.001), and cerebellum (55%, P <.001). In conclusion, the applicability of the flooding dose technique to measure protein synthesis in the brain in vivo is demonstrated by its ability to measure regional difference. Impaired protein synthesis rates may contribute to or reflect the pathogenesis of alcohol-induced brain damage.

Brain nucleic acid composition and fractional rates of protein synthesis in response to chronic ethanol feeding: comparison with skeletal muscle

Brain atrophy is a common feature of chronic alcohol misuse, although the pathogenic mechanisms are unknown. We propose that defects in protein synthesis are contributing events. To test this hypothesis the experimental effects of chronic (i.e., 2 and 3 weeks) ethanol feeding on brain nucleic acid composition and rates of protein synthesis in vivo were investigated. These were compared with those of skeletal muscle (represented by the plantaris). Male Wistar rats, used at mean body weights of either 82 g (first study for 2 weeks ) or 93 g (second study for 3 weeks) were fed a nutritionally complete liquid diet in which ethanol comprised a third of the total calories. Control rats were pair-fed identical amounts of the same diet, in which ethanol was substituted by isoenergetic glucose. At 2 weeks there were small reductions (i.e., approximately 5-10%) in the weight of the whole brain, cortex, and brain stem. Ethanol-induced reductions in the total protein content of the brain stem was found at 2 weeks, although these changes did not achieve significance. At 3 weeks the weights of whole brain were significantly reduced compared to a greater reduction in skeletal muscle weights. Total protein contents were reduced at 3 weeks in the whole brain and skeletal muscle. At 2 weeks there were decreases in the RNA contents of the cortex, brain stem, and entire brain. There were also reductions in cerebellum RNA composition only when expressed relative to DNA. The DNA composition of the brain was relatively unaffected by chronic ethanol feeding. At 3 weeks, total RNA and DNA were reduced in the whole brain and muscle. Fractional rates of protein synthesis (i.e., the percentage of tissue protein pool renewed each day) in the brain were unaltered after 3 weeks of ethanol feeding, but were reduced in skeletal muscles, largely as a consequence of reduced RNA composition. In conclusion, only moderate changes in the brain were found in ethanol feeding. These data can be compared to skeletal muscle, which shows that ethanol induces profound reductions in protein, RNA, and protein synthesis rates.

Lysosomal and nonlysosomal protease activities of the brain in response to ethanol feeding

The contribution of impaired degradative processes to the cellular changes occurring in the brain as a consequence of chronic ethanol exposure was assessed. Male Wistar rats were fed nutritionally adequate liquid diets containing ethanol as 35% of total dietary calories. Controls were pair-fed identical amounts of the same diet in which ethanol was replaced by isocaloric glucose. The results showed that at the end of 3 weeks the activities of neutral protease (nonlysosomal) and cathepsin D (lysosomal) were unaltered. However, there were significant elevations in the activities of the lysosomal enzyme cathepsin B, regardless of whether the activities were expressed relative to wet weight ( p = 0.005), protein (p = 0.006), or DNA (p = 0.045). In addition, we showed that the activities of cathepsin B were not significantly affected by additions of carnosine or acetaldehyde, in vitro. However, neutral protease activities were increased by carnosine additions in vitro. We conclude that selective alterations in brain protease activities may be contributing factors in the genesis of alcoholic brain disorders.

Changes in activities of some ammonia-metabolizing enzymes in the rat liver and the brain after chronic ethanol administration

The changes in the activities of ammonia-metabolizing enzymes in liver and brain after ethanol intoxication has been investigated in rats. After administration of ethanol 30% (w/v) 6g kg-1 for 4 weeks we found an increase in liver glutamate dehydrogenase and glutaminase activity. In brain tissue the glutaminase activity was significantly higher and glutamate dehydrogenase was significantly lower. Glutamine synthetase activity in liver and brain was practically unchanged. The reasons for these changes in the activities of some ammonia-metabolizing enzymes in liver and brain after ethanol ingestion have been discussed.