Measurements of in vivo rates of protein synthesis in brain, spinal cord, heart and liver of young versus adult rats, intact versus hypophysectomized rats

Turnover of amyloid beta-protein in mouse brain and acute reduction of its level by phorbol ester

Fibrillar amyloid deposits are defining pathological lesions in Alzheimer's disease brain and are thought to mediate neuronal death. Amyloid is composed primarily of a 39-42 amino acid protein fragment of the amyloid precursor protein (APP), called amyloid beta-protein (Abeta). Because deposition of fibrillar amyloid in vitro has been shown to be highly dependent on Abeta concentration, reducing the proteolytic release of Abeta is an attractive, potentially therapeutic target. Here, the turnover rate of brain Abeta has been determined to define treatment intervals over which a change in steady-state concentration of Abeta could be measured. Mice producing elevated levels of human Abeta were used to determine approximate turnover rates for Abeta and two of its precursors, C99 and APP. The t1/2 for brain Abeta was between 1.0 and 2.5 hr, whereas for C99, immature, and fully glycosylated forms of APP695 the approximate t1/2 values were 3, 3, and 7 hr, respectively. Given the rapid Abeta turnover rate, acute studies were designed using phorbol 12-myristate 13-acetate (PMA), which had been demonstrated previously to reduce Abeta secretion from cells in vitro via induction of protein kinase C (PKC) activity. Six hours after intracortical injection of PMA, Abeta levels were significantly reduced, as measured by both Abeta40- and Abeta42-selective ELISAs, returning to normal by 12 hr. An inactive structural analog of PMA, 4alpha-PMA, had no effect on brain Abeta levels. Among the secreted N-terminal APP fragments, APPbeta levels were significantly reduced by PMA treatment, whereas APPalpha levels were unchanged, in contrast to most cell culture studies. These results indicate that Abeta is rapidly turned over under normal conditions and support the therapeutic potential of elevating PKC activity for reduction of brain Abeta.

Brief fasting decreases protein synthesis in the brain of adult rats

The influence of starvation on protein synthesis in the adult rat brain was studied in vivo by an intravenous injection of a flooding dose of unlabeled valine including a tracer dose of L-[3,4(n)-3H]valine. Brief starvation (24 hours) induced a 20% decline in fractional and absolute rates of brain protein synthesis. This decline resulted from a 20% decrease in the efficiency of protein synthesis (microgram protein synthesized per day per microgram RNA) whereas the capacity for protein synthesis (microgram RNA per mg protein) was maintained. Prolonged starvation (5 days) was marked by no further significant changes in the fractional rate, absolute rate and efficiency of protein synthesis, whereas the capacity for protein synthesis decreased slightly. The relative contribution of brain to whole-body protein synthesis increased during fasting, and neither the protein nor the RNA brain content did change during the experiment. These results clearly indicate that brain proteins are spared in response to brief and prolonged food deprivation, and that brain protein synthesis is very sensitive to short-term fasting.

In vivo rates of protein synthesis in brain, muscle, and liver of five vertebrate species

To compare cerebral protein metabolism rates in vivo, protein synthesis rates of three organs of five vertebrate species were measured after a single i.p. injection of a flooding dose of [1-14C]valine. In muscle, brain, and liver, the respective average protein synthesis rates, expressed as percent of total protein-bound valine replaced per hour, that is, percent synthesis per hour, in goldfish at 22 degrees C body temperature, were 0.07, 0.23, and 0.57%; in the bullfrog at 20 degrees C, 0.06, 0.18, and 0.55%; in the white Leghorn chicken at 39 degrees C, 0.24, 0.70, and 2.17%; and in the mouse at 38 degrees C, 0.22, 0.65, and 2.0%. In the Tokay lizard at different body temperatures, the synthesis rates were 0.04, 0.13, and 0.43% at 26 degrees C; 0.05, 0.20, and 0.63% at 32 degrees C; and 0.07, 0.27, and 0.81% at 38 degrees C. The results demonstrate differences in protein synthesis rates in organs of the various species examined. The differences among the species seem to be due, to a major extent, to differences in body temperature; rates in lizard are below those in other species at temperatures tried. Protein synthesis rates in brain in all species are almost three times lower than those in liver and almost three times higher than those in muscle.

Protein synthesis rates in rat brain regions and subcellular fractions during aging

In vivo protein synthesis rates in various brain regions (cerebral cortex, cerebellum, hippocampus, hypothalamus, and striatum) of 4-, 12-, and 24-month-old rats were examined after injection of a flooding dose of labeled valine. The incorporation of labeled valine into proteins of mitochondrial, microsomal, and cytosolic fractions from cerebral cortex and cerebellum was also measured. At all ages examined, the incorporation rate was 0.5% per hour in cerebral cortex, cerebellum, hippocampus, and hypothalamus and 0.4% per hour in striatum. Of the subcellular fractions examined, the microsomal proteins were synthesized at the highest rate, followed by cytosolic and mitochondrial proteins. The results obtained indicate that the average synthesis rate of proteins in the various brain regions and subcellular fractions examined is fairly constant and is not significantly altered in the 4 to 24-month period of life of rats.

Nicotine-induced changes in the metabolism of specific brain proteins

The effect of acute and chronic nicotine on the metabolism of specific brain proteins was examined by measuring incorporation of labeled valine into protein, with densitometric scanning of proteins resolved by gel electrophoresis. Acute and chronic administration of nicotine (0.4 mg/kg per 30 min for 2 hours, s.c., or 0.5 mg/kg per 30 min for 5 days (Alzet mini-pump implanted subcutaneously] reduced incorporation of [14C]valine administered by approximately 6-7%. The results with chronic nicotine administration indicated a lack of tolerance for this effect of nicotine. Mecamylamine, a nicotinic ganglionic antagonist, does not seem to block the inhibition of protein synthesis. Small increases in protein content were observed in a high- and a low-molecular-weight region of SDS-polyacrylamide gel, used to separate proteins from newborn brain. In adult brain after chronic nicotine administration, selective increases and a decrease were seen in selective bands. Results are consonant with selective effects of nicotine on the synthesis or degradation of specific brain proteins.

Inhibition of protein synthesis by trimethyltin

The effects of trimethyltin chloride (TMT) on protein synthesis, measured as the incorporation of [3H]valine into trichloroacetic acid-precipitable material, were investigated in mice. One hour after intraperitoneal administration of a 3.0 mg/kg dose, TMT decreased brain protein synthesis by 47% and also caused a significant decrease (4.2 degrees C) in body temperature. When hypothermia was prevented by maintaining the animals at 35 degrees C, TMT decreased protein synthesis by 20%. Twenty-four hours following administration of TMT, protein synthesis was decreased in brain and liver; however, only a reduction of brain protein synthesis was observed at 48 hr. No hypothermia was present at either time point. A regional study in brain showed that at 24 and 48 hr after TMT administration, protein synthesis was decreased by 18-23% in cerebral cortex and hippocampus but not in cerebellum. TMT also inhibited protein synthesis in vitro in mouse brain homogenates with an IC50 of about 100 microM. Neither SnCl2, nor dimethyltin or monomethyltin had any effect on protein synthesis in vitro. These results suggest that, as for other neurotoxicants such as methyl mercury or acrylamide, inhibition of protein synthesis might be involved in TMT neurotoxicity.

Effect of diethylmaleate and other glutathione depletors on protein synthesis

The alpha, beta-unsaturated carbonyl compound diethylmaleate (DEM) depletes glutathione (GSH) from liver and other tissues, and for this reason it is often used in toxicological research to study the GSH-mediated metabolism of xenobiotics. In addition to GSH depletion, however, DEM has been shown to have other nonspecific effects, such as alteration of monooxygenase activities or glycogen metabolism. In this study we found that DEM (1 ml/kg) inhibited protein synthesis in brain and liver, following in vivo administration to mice. Protein synthesis was measured as the incorporation of [3H]valine into trichloroacetic acid-precipitable material. Administration of DEM also decreased body temperature by 2-3 degrees. By increasing the environmental temperature from 22 degrees to 35 degrees the hypothermic effect of DEM was prevented, without affecting its ability to deplete GSH from brain and liver. Furthermore, when mice were maintained at 35 degrees, DEM still caused a significant decrease in protein synthesis, suggesting that this effect was only partially due to hypothermia. To test whether inhibition of protein synthesis was related to GSH depletion, groups of animals were dosed with the alpha, beta-unsaturated carbonyl phorone (diisopropylidenacetone) or the specific inhibitor of GSH synthesis, buthionine sulfoximine (BSO). Phorone decreased GSH in liver and brain; however, it had no effect on protein synthesis. BSO decreased GSH levels in liver and kidney, but not in brain, and did not have any effect on protein synthesis in any of these tissues, nor did it cause any hypothermia. Furthermore, when hepatic GSH content was decreased by in vivo administration of DEM or BSO, there was no inhibition of protein synthesis measured in vitro. These results indicate that, at the dose normally used to deplete GSH from various tissues. DEM also exerts an inhibitory effect on protein synthesis, which appears to be only partially due to its hypothermic effect, and is independent from GSH depletion. BSO, which, in our experimental conditions, lacks this and other nonspecific effects, might be a good alternative for studies aimed at characterizing the role of GSH in the metabolism and toxicity of chemicals.

Triple-tracer autoradiography of cerebral blood flow, glucose utilization, and protein synthesis in rat brain

A triple-tracer autoradiographic technique is described that permits the simultaneous measurement of cerebral blood flow, glucose consumption, and protein synthesis using 131I-iodoantipyrine (131I-IAP), [14C]deoxyglucose ([14C]DG), and 3H-amino acids as radioactive tracers. Autoradiographic differentiation between isotopes was performed by taking advantage of different half-lives, solubility of labeled tracers in a wash solution, and sensitivity of the photographic material to disintegrations of the radionuclides. Blood flow autoradiograms using 131I-IAP were obtained by immediate exposure of brain sections to Kodak NMB film for 24 h. During 131I autoradiography contamination by 3H was absent and by 14C was negligible at tissue concentrations of less than 0.45 microCi/g brain tissue. After complete decay of 131I, reexposure of brain sections to Kodak NMB film for 2 weeks provided autoradiograms that stemmed exclusively from 14C disintegrations without contamination by either 131I or 3H and that represented regional glucose utilization. Brain sections were then wash-incubated for 12 h to remove [14C]DG, [14C]DG-6-phosphate, and free 3H-amino acids from the tissue, and exposed to 3H-sensitive LKB Ultrofilm for 2 weeks for autoradiography of 3H-amino acid incorporation into proteins. 14C radioactivity remaining in the tissue section after wash-incubation was determined by exposing sections again for 2 weeks to Kodak NMB film; the resulting contribution to the blackening of 3H-autoradiograms was corrected for by means of digital subtraction using an image-processing system. The triple-tracer autoradiographic technique was validated in rats under various physiological and pathophysiological conditions. In intact animals extinction correction was necessary only for 3H-autoradiograms. Under pathophysiological conditions, however, significant contamination of 131I by 14C occurred in regions with low blood flow and increased glucose utilization rate; this also required correction by digital subtraction. The interpretation of triple-tracer autoradiographic results is limited by the same restrictions as single-tracer autoradiography, but the simultaneous assessment of the three parameters considerably facilitates the interpretation of the flow/metabolic relationship, particularly under pathological conditions.

Regionally selective inhibition of cerebral protein synthesis in the rat during hypoglycemia and recovery

Regional cerebral protein synthesis was investigated in anesthetized, mechanically ventilated rats during progressive insulin-induced hypoglycemia and the recovery period following glucose infusion. Polysome profiles from precomatose animals with slow wave/polyspike EEG revealed a slight reduction of polyribosomes and a concurrent increase in monoribosomes, but autoradiographs showed a pattern of L-[3-3H]tyrosine incorporation indistinguishable from that of control rats. During the initial 30 min of insulin-induced isoelectric EEG ("coma"), autoradiographs showed a selective inhibition of protein synthesis in neurons and glial cells of the hippocampus and cerebral cortex, i.e., regions with high susceptibility for the development of hypoglycemic brain damage. Basal ganglia were less affected and areas with low vulnerability (hypothalamus, brainstem, and cerebellum) exhibited a normal pattern of amino acid incorporation. Using a flooding dose of L-[1-14C]valine (7.5 mmol/kg; 15 microCi/mmol), the rate of incorporation in cerebral cortex and cerebellum was found to be reduced to 2% and 80% of control values, respectively. Inhibition of protein synthesis was paralleled by a breakdown of polyribosomes and a concomitant increase in ribosomal subunits, indicating a block in peptide chain initiation. After 90 min of isoelectric EEG all brain structures with the exception of hypothalamus and area postrema showed an almost complete lack of amino acid incorporation. Glucose infusion after a 30-min period of hypoglycemic coma led to a partial restoration of cortical and hippocampal protein synthesis. Within 70-90 min of recovery, L-[1-14C]valine incorporation into neocortical and cerebellar proteins amounted to 47% and 125% of fasted controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of brain ornithine decarboxylase during recovery from metabolic, mechanical, thermal, or chemical injury

Metabolic, mechanical, thermal, and chemical injury induced ornithine decarboxylase (ODC) activity in rat brain. A two- to sixfold increase in ODC activity was measured at 5-9 h after different modes of injury to the brain. During the early phase of recovery from transient ischemia, when average protein synthesis was less than 50% of control, ODC activity was increased nearly fivefold. The rise in activity could be blocked by anisomycin, or reduced by intracerebral injections of actinomycin D. Drilling burr holes into the skull, injection of the vehicle for actinomycin D, hyperthermia, and freezing lesions all caused increased ODC activity. Neurotoxic chemicals (ammonia, methionine sulfoximine, acrylamide, carbon tetrachloride, and anisomycin) also increased brain ODC activity, whereas other chemicals (mannitol and valine) did not. Treatments known to stimulate the synthesis of heat shock proteins (carotid occlusion, hyperthermia, Cd2+, canavanine, and ethanol) induced ODC activity in the liver, whereas only hyperthermia and ethanol caused significant increases in spleen ODC activity. All increases in ODC activity were blocked by difluoromethylornithine, an irreversible inhibitor of ODC. The cellular response to noxious or stressful stimuli includes the synthesis of a small number of proteins of unknown functions; ODC may be one of these "heat shock" or "trauma" proteins.

Regional impairment of protein synthesis in the rat brain during bicuculline-induced seizures

Protein synthesis was investigated in rats subjected to 30 min of bicuculline-induced seizures using biochemical and autoradiographic techniques. Incorporation studies were performed on freely convulsive animals following systemic administration of either a tracer dose of L-[1-14C]tyrosine or a flooding dose (7.5 mmol/kg) of L-[1-14C]valine. Using a tracer dose, amino acid incorporation was only moderately reduced (forebrain) or slightly enhanced (cerebellum/brainstem and spinal cord) but the specific radioactivity of [14C]tyrosine in the acid-soluble pool was increased 3- to 5-fold in experimental animals. After a flooding dose of [14C]valine the specific activity of the precursor amino acid was similar in control and convulsed animals. Under these conditions incorporation rates in forebrain and cerebellum/brainstem were reduced to 54 and 75%, respectively. Reduction of amino acid incorporation was even more pronounced in extraneural tissues, e.g. liver (6%), intestine (14%) and kidney (15%). Inhibition of protein synthesis in forebrain and cerebellum/brainstem was paralleled by a similar extent of polyribosome disaggregation in these regions (53 adn 78% of controls). In anaesthetized, mechanically ventilated rats, 30 min of seizure activity reduced forebrain polyribosomes to a similar extent (57%). Extraneural (hepatic) protein synthesis was also affected in physiologically controlled rats, but cerebellar polysomes were completely preserved. Autoradiographic studies using 3H-labelled amino acids were carried out to identify nerve cell populations most heavily affected. In freely convulsive rats both tracer dose and pool overloading revealed a similar regional pattern with preferential inhibition of amino acid incorporation in forebrain cortex, thalamus and the pyramidal cell layer of the hippocampus. These sites were also affected in the physiologically controlled animal, but the focal distribution of hippocampal and thalamic neurones with reduced protein synthesis differed from that in freely convulsive rats.

No effect of acute ethanol administration on hepatic protein synthesis and export in the rat in vivo

Ethanol was administered as a single p.o. dose (2.88 g X kg-1) to male rats (220-265 g body weight) to give blood alcohol concentrations of 40-50 mM for the following 3 hr. Controls were given isoenergetic amounts of either sucrose or lipid. Liver protein synthetic rates were measured during a 20 min interval at the end of the 3 hr period following the administration of diets. Although ethanol caused a 32% reduction of the incorporation of labelled valine into liver protein compared to the sucrose group during the 20 min interval, no such reduction was found when the synthetic rate of stationary liver protein was calculated (182 vs. 214 (not significant) pmol X mg protein-1 X min-1) for same interval. There was no difference between the ethanol and lipid group with regard to either incorporation or synthetic rates. Incorporation of valine into plasma proteins was reduced in the ethanol group compared to the sucrose group, but not compared to the lipid control group, again demonstrating no ethanol-specific effect. When the incorporation into plasma proteins was divided by the specific radioactivity of valyl-tRNA at 20 min, the difference between the ethanol and the sucrose group disappeared. The fraction of newly synthesized proteins exported to the plasma measured 40 min after the injection of labeled valine, was equal in all three treatment groups. It was concluded that acute administration of ethanol has no consistent effect on liver protein synthesis and secretion in vivo.

Rates of protein synthesis and degradation in Gunn rat cerebellum with bilirubin-induced cerebellar hypoplasia

The rates of protein synthesis and degradation, and the endogenous leucine content on days 13 and 30, were examined in the cerebellum of homozygous Gunn rats with cerebellar hypoplasia and compared with those of heterozygotes. The rate of protein synthesis based on milligrams of protein was significantly lower on day 30 and the degradation rate was about ten times faster between days 13 and 18. Furthermore, leucine concentration was about 1.7 times higher on day 13. The present results suggest that the lowered protein concentration previously observed in the cerebellum of homozygous Gunn rats is ascribed to the increased rate of protein degradation on day 13, and to the decreased rate of protein synthesis on day 30.

A Quantitative Regional Analysis of Amino Acids Involved in Rat Brain Protein Synthesis by High Performance Liquid Chromatography

A biochemical method is described for the simultaneous quantitative estimation of unidirectional blood-brain amino acid influx and protein biosynthesis in individual structures of the rat brain. The method involved a double labeling experiment started by the administration of [14C]carboxyl-labeled amino acids and terminated 2 min after infusion of 3H-labeled amino acids, each at tracer quantities, the total labeling period being 45 min. Specific radioactivities of 14C- or 3H-labeled phenylalanine, tyrosine, leucine, isoleucine, and valine were determined in plasma and in small brain tissue samples for free amino acids, aminoacyl-tRNAs, and proteins. Amino acids were converted to their corresponding 5-dimethylamino-naphthalenesulfonyl (Dns, dansyl) derivatives and separated on HPLC C18 reversed-phase columns isocratically according to a newly developed optimizing procedure. The order of influx values between the neutral amino acids in relation to each other was Leu greater than Tyr greater than Ile greater than Phe greater than Val in every structure examined. Although aminoacylation of tRNAs was found to proceed to a comparable degree for neutral amino acids in all regions investigated, the specific radioactivity of amino acids attached to tRNAs differed substantially from that in the free amino acid pool, especially for leucine and valine. The results indicate the necessity of aminoacyl-tRNA determinations for tracer incorporation studies in protein synthesis analysis. Relative protein synthesis rates in the halothane-anesthetized rat were determined to be 30 and 67-91 pmol total amino acid incorporation/min/mg tissue for white and gray matter, respectively.

A quantitative autoradiographic method for the measurement of local rates of brain protein synthesis

We have developed a new method for measuring local rates of brain protein synthesis in vivo. It combines the intraperitoneal injection of a large dose of low specific activity amino acid with quantitative autoradiography. This method has several advantages: 1) It is ideally suited for young or small animals or where immobilizing an animal is undesirable. 2 The amino acid injection "floods" amino acid pools so that errors in estimating precursor specific activity, which is especially important in pathological conditions, are minimized. 3) The method provides for the use of a radioautographic internal standard in which valine incorporation is measured directly. Internal standards from experimental animals correct for tissue protein content and self-absorption of radiation in tissue sections which could vary under experimental conditions.

In vivo effect of methylmercury on protein synthesis in peripheral nervous tissues of the rat

The in vivo rates of protein synthesis in the peripheral nervous tissues of methylmercury-treated rats (10 mg/kg/day, for 7 days) have been estimated with improved methods by the injection of a large amount of [1-14C]valine of low specific activity. Protein synthesis activity in the dorsal root ganglia was inhibited to the extent of 60% of the control as early as day 5 and this continued to the symptomatic period (day 15) on which crossing of hind limbs, a typical sign of organomercurial poisoning, was observed in the animals. The sciatic nerves and dorsal roots increased protein synthesis by 56% at the symptomatic period. These increases in protein synthesis may be due to the stimulation of reactivity of Schwann's cells. On the contrary, the protein synthesis in the ventral roots showed a gradual decrease as the intoxication proceeded and decreased to 73% of the control at the symptomatic period, being similar to the case of brain. The double-labeling studies with sodium dodecyl sulfate/polyacrylamide gel electrophoresis exhibited that methylmercury inhibited the synthesis of the dorsal root ganglion proteins non-uniformly in various apparent molecular sizes, especially on day 10.

Changes in in vivo rates of protein synthesis on free and membrane-bound polysomes in rat brain during the development of physical dependence on ethanol and after the withdrawal of ethanol

Administration of ethanol and nutrients to rats thrice daily by gavage for 3 days produced a linear increase in physical dependence during the first 3 days and a 2% increase in body weight. Rates of protein synthesis on free and membrane-bound polysomes in whole brain and in 7 brain regions, comprising the entire brain, were measured in vivo under pool expansion conditions with [3H]leucine at intervals during the development and decay of ethanol dependence. During dependence development there was a progressive decrease in the rate of protein synthesis on free polysomes, but this change was not significant (P less than 0.05) until the third day, and a decrease in the rate on membrane-bound polysomes after 3 days. The inhibition of protein synthesis is attributable to a decreased rate of polypeptide elongation. There was no change in brain weight, DNA content, ribosome content, ribosome distribution of mRNA pool size. There was, however, a decrease in leucine uptake after 3 days. In an attempt to distinguish between the acute effects of ethanol on regional rates of protein synthesis and those changes associated with dependence development, rates were measured 1.5 h after administering a 5 g/kg dose of ethanol to both control and dependent rats. Rates on free polysomes in the hippocampus-amygdala and thalamus-hypothalamus and on membrane-bound polysomes in the cerebellum and hippocampus-amygdala of dependent rats were reduced; however, there was a general reduction in the rates in control rats that may have obscured reductions in other regions from dependent rats. During early withdrawal, 12 h after the last dose of ethanol, there was an increase in the rate of free polysomes in the pons-medulla and striatum-septum and on membrane-bound polysomes in the hippocampus-amygdala, and a decrease in the rate on free polysomes in the cortex and thalamus-hypothalamus and on membrane-bound polysomes in the cortex. After 24 h, there was an increase in the rate on free polysomes in all regions (cerebellum, cortex, mesencephalon, striatum-septum and thalamus-hypothalamus) except the hippocampus-amygdala and pons-medulla and an increase in the rate on membrane-bound polysomes in all regions (cortex, hippocampus-amygdala, mesencephalon, pons-medulla and striatum-septum) except the cerebellum and thalamus-hypothalamus. The possible relationship of these changes to the homeostat hypothesis of ethanol dependence is discussed.

Pineal protein synthesis highly sensitive to ACTH-like neuropeptides

Pineal protein synthesis was studied in vitro over a period of 6-8 h after dissection. The level of protein synthetic activity of the pineal gland was greatly dependent on the time of dissection showing a maximum at midnight and a minimum at 10.00 h, 2 h after onset of light. Low concentrations of ACTH1-24 (down to 10(-11) M) could stimulate protein synthesis in vitro. The sensitivity to hormonal stimulation showed a circadian variation similar to that observed in the basal protein synthetic activity. Furthermore, overall synthetic activity appeared to be under neural influence. These neural and hormonal influences seemed to be mediated by beta-receptor stimulation and cyclic AMP. Structure-activity studies of the ACTH-effect on pineal protein synthesis gave results similar to those previously observed for excessive grooming behaviour, synaptic plasma membrane phosphorylation, adenylcyclase-activity and cell-free protein synthesis in brain. It was concluded, that overall pineal protein synthesis is both under neural and hormonal control. The action of ACTH on protein synthesis rate might be mediated by a calcium-dependent release of norepinephrine followed postsynaptically by beta-receptor activation, cAMP production, and stimulation of translation.

Stimulation of fructose-1,6-biphosphatase activity and synthesis in the cerebral cortex of rats submitted to the convulsant methionine sulfoximine

Purification of rat cerebral cortex fructose-1,6-biphosphatase (FBPase) was performed by substrate elution from phosphocellulose, followed by Sephadex G-200 column filtration. The purified enzyme exhibited an optimum at pH 7.5, and its catalytic properties were very similar to those of the purified whole-brain enzyme previously prepared by Majumder and Eisenberg in 1977. The isolated preparation was electrophoretically homogeneous. The molecular weight of the enzyme subunit was 40,000; the hydrophobic amino acids predominated with 592 residues, and tryptophan was not detected. Expressed as mumol fructose-1,6-biphosphate hydrolysed per g brain tissue wet weight per min, FBPase activity increased twofold 24 h after an intraperitoneal injection of 100 mg per kg body weight of the convulsant methionine sulfoximine (MSO); the increase of the rate of incorporation of [1-14C]valine into brain FBPase was 2.8-fold under the same experimental conditions. A rabbit specific antiserum against rat cerebral cortex FBPase was prepared, and immunotitration studies confirmed both an increase in the number of molecules and the activation of brain FBPase, 24 h after administration of MSO. The increase of the number of brain FBPase molecules, induced by MSO, was due to an increase in synthesis of the enzyme, as shown by a double-label valine incorporation study.

Regional protein synthesis in the rat brain during bicuculline-induced epileptic seizures

The incorporation of L-[3,5-3H]tyrosine into cerebral proteins was investigated during the initial phase (30 min) of bicuculline-induced status epilepticus. Autoradiographs of different parts of the cerebral hemispheres, brain stem, and cerebellum were prepared. Marked local reduction of amino acid incorporation was evident in bilaterally symmetrical areas of the cerebral cortex, hippocampus, thalamic nuclei, and the region of the medial geniculate body. No apparent difference of local [3H]tyrosine incorporation was observed in the lower brain stem nuclei and in the cerebellum of control and convulsed animals. The territories showing a decrease of protein synthesis during epileptic seizures coincide largely with the regions of maximal local glucose metabolism and cerebral blood flow. The present investigation demonstrates that autoradiography of regional protein biosynthesis is a suitable method for the visualization of neuronal populations at risk in the very early stages of seizure activity.

Regional protein synthesis in rat brain following acute hemispheric ischemia

Regional protein synthesis was measured in rat brain at intervals up to 48 h following occlusion of the four major arteries to the brain for either 10 or 30 min. Four-vessel occlusions produces ischemia in the cerebral hemispheres and oligemia in the midbrain-diencephalon and brainstem. During the hour following 10 min of ischemia, protein synthesis, measured by incorporation of [14C]valine into protein, was inhibited in the cerebral cortex by 67%. Normal rates of protein synthesis were attained within 4 h of recirculation. In rats subjected to 30 min of ischemia, protein synthesis was inhibited by 83% during the first hour of recirculation in the cortex, caudate-putamen, and hippocampus. Recovery of protein synthesis in these regions was slow (25-48 h). The midbrain-diencephalon showed less inhibition, 67%, and faster recovery (by 12 h). Protein synthesis was unaffected in the brainstem. [14C]Autoradiography revealed that the pyramidal neurons of the hippocampus and areas of the caudate and cortex failed to recover normal rates of protein synthesis even after 48 h. The accumulation of TCA-soluble [14C]valine was enhanced (55-65%) in the cortex, caudate, and hippocampus after 30 min of ischemia; the increase persisted for 12 h. A smaller rise in [14C]valine content (30%) and more rapid normalization of valine accumulation (by 7 h) were observed in the midbrain-diencephalon; no changes were found in the brainstem. In the cortex, recovery was more rapid when the duration of ischemia was reduced. Thus, the degree of inhibition of protein synthesis, the accumulation of valine in the tissue, and the length of time required to reestablish normal values for these processes were dependent on both the severity and the duration of the ischemic insult. Restoration of normal rates of protein synthesis after ischemia was slow compared with the normalization of cerebral energy metabolites.

Rat brain protein synthesis declines during postdevelopmental aging

Using improved methods to measure brain protein synthesis in vivo (Dunlop et al., 1975) we have established that brain protein synthesis significantly declines in forebrain, cerebellum and brain stem when mature rats (3 months old) are compared to old rats (22.5 months old). The incorporation of (3H) L-lysine into forebrain protein is reduced 11% in 10.5 month old rats compared to 3 month old rats. A further reduction of 9% occurred between 16.5 months and 22.5 months. Our data suggest that reduced levels of protein synthesis initiation may be responsible, at least in part, for this age-related decline.

Age-dependent changes in brain protein synthesis in the rat

Brain protein synthesis was studied in vivo, in brain slices, and in cell-free systems in rats aged 1, 16, and 24 months. We observed a highly significant reduction in amino acid incorporation with advancing age. This reduction was observed in vivo, in slices, in postmitochondrial supernatant, microsomes, and membrane-bound polysomes. Free heavy polysomes showed no age-dependent decline but formed a smaller proportion of total ribosomes in older animals. These studies suggest that in the rat brain protein synthesis declines before senescence, possibly due to an impairment in the initiation process.

A simple reproducible cell-free system for measuring brain protein synthesis

A simple, rapid, sensitive, and reproducible cell-free assay system for studying brain protein synthesis is described. This system uses small amounts of brain postmitochondrial supernatant, making it a convenient screening test when only small amounts of tissue are available. It showed over 95% dependence on Mg2+ and on an energy source. Optimal incorporation occurred under the following conditions: Mg2+ 3 mM; ATP, 0.6 mM; GTP, 0.6 mM; high K+, greater than or equal to 25 mM; Low Na+, less than or equal to 15 mM; pH 7.1-7.5. The rate of amino acid incorporation did not vary with leucine concentrations in vitro up to 1 mM, which obviated the need to measure endogenous leucine concentrations.

Isolation and characterization of free and membrane-bound polyribosomes from rabbit spinal cord

A procedure is described for the preparation of free and membrane-bound polyribosomes from rabbit spinal cord. First, myelin and filaments are floated away from other subcellular components. The latter are then fractionated by differential centrifugation followed by sedimentation through a discontinuous sucrose gradient. The ribosomal fractions are characterized by their electron microscopic appearance, RNA/protein ratios and sedimentation profile in a linear sucrose gradient. Both membrane-bound and free polyribosomes are active in incorporating amino acids in a cell-free system, whether heterologous or homologous pH 5 enzyme fractions are employed. Autoradiographic analysis of translation products separated by electrophoresis on SDS-polyacrylamide gels shows that both ribosome fractions are active in the synthesis of a variety of proteins including polypeptides which comigrate with alpha- and beta-tubulins and actin. The utility of these polyribosome preparations for the cell-free study of protein biosynthesis is indicated by the high molecular weight of many of the translation products, and by the similarity of the electrophoretic pattern of translation products from the cell-free systems to the pattern of radioactive protein synthesized by rabbit spinal cord during the hour prior to sacrifice.

Compartments of protein metabolism in the developing brain

We investigated whether the higher rate of amino acid incorporation into immature than into mature brain protein is due to (a) rapid growth, (b) a small rapidly metabolized protein pool, or (c) a higher turnover rate of most of the protein. We measured net growth and the incorporation of [14C]tyrosine or [14C]valine into brain proteins in young rats and mice. The specific activity of the free amino acid pool was kept constant in the tyrosine experiments. Incorporation of tyrosine into protein was continued for up to 30 h by which time the specific activity of protein-bound amino acid reached 1/3 of that of the free (precursor) amino acid. The growth (accretion) of brain proteins was approx. 0.635% per h in mice and rats in the 1-4 day period after birth. In previous studies we found that the turnover rate of the bulk (about 96%) of adult brain proteins is below 0.3% per h. Because of the presence of a small (about 4%) active pool the average turnover rate is 0.6% per h. The present experiments show a degradation rate of 0.7-1.1% per h in the brain proteins of the young. This high metabolic rate is not due to a small rapidly degraded fraction of protein. The very rapid protein fraction previously seen in adult rats is either very small (below 1%) or absent in the young. Thus most of the proteins in the immature brain during the rapid growth phase are formed and broken down at a rate that is approximately three times higher than that of the bulk of proteins in the adult brain. The small active protein pool in the adult on the other hand has a metabolic rate higher than that of the immature brain proteins.