Measurement of brain protein turnover with [14C]sodium bicarbonate

Reduced clearance of proteins labeled with diisopropylfluorophosphate in portacaval-shunted rats

Portacaval shunting is a model for hepatic encephalopathy that causes chronic hyperammonemia, disruption of metabolic, signaling, and neurotransmitter systems, and progressive morphological changes. Exposure of cultured cells to ammonia raises intralysosomal pH and inhibits proteolysis, and the present study tested the hypothesis that proteolytic capacity is diminished in portacaval-shunted rats. Proteins were labeled in vivo with tracer doses of diisopropylfluorophosphate (DFP) and clearance of label was assayed. This approach labeled proteins independent of protein synthesis, which is reported to be altered in shunted rats, and avoided complications arising from re-utilization of labeled amino acids that causes underestimation of degradation rate. Characterization of DFP labeling showed that protein labeling was fast, about 50% of the label was released during a 24 h interval, labeling by DFP metabolites was negligible, inhibition of brain acetylcholinesterase was not detectable, and labeling by [(3)H]- and [(14)C]DFP was equivalent. To assay degradative capacity, proteins were first labeled with [(3)H]DFP, followed by labeling with [(14)C]DFP that was given 24 or 72 h later. The (3)H/(14)C ratio in each animal was used as a relative measure of removal of (3)H-labeled proteins. (3)H/(14)C ratios were generally significantly higher in portacaval-shunted rats than in controls, consistent with reduced proteolytic capacity. Assays of amino acid incorporation into brain protein generally replicated literature reports, supporting the conclusion that protein synthesis unlikely to be markedly inhibited and amino acid recycling influences calculated protein synthesis rates in shunted rats. Therapeutic strategies to reduce ammonia level would help normalize lysosomal functions and protein and lipid turnover.

Brain protein synthesis in the conscious rat using L-[35S]methionine: relationship of methionine specific activity between plasma and precursor compartment and evaluation of methionine metabolic pathways

The method previously developed for the measurement of rates of methionine incorporation into brain proteins assumed that methionine derived from protein degradation did not recycle into the precursor pool for protein synthesis and that the metabolism of methionine via the transmethylation pathway was negligible. To evaluate the degree of recycling, we have compared, under steady-state conditions, the specific activity of L-[35S] methionine in the tRNA-bound pool to that of plasma. The relative contribution of methionine from protein degradation to the precursor pool was 26%. Under the same conditions, the relative rate of methionine flux into the transmethylation cycle was estimated to be 10% of the rate of methionine incorporation into brain proteins. These results indicate the following: (a) there is significant recycling of unlabeled methionine derived from protein degradation in brain; and (b) the metabolism of methionine is directed mainly towards protein synthesis. At normal plasma amino acid levels, methionine is the amino acid which, to date, presents the lowest degree of dilution in the precursor pool for protein synthesis. L-[35S]-Methionine, therefore, presents radiobiochemical properties required to measure, with minimal underestimation, rates of brain protein synthesis in vivo.

Altered cerebral protein turnover in rats following prolonged in vivo treatment with nicotine

Turnover rates of cerebral proteins were examined in control adult rats and in those subjected to prolonged in vivo treatment with "low" (0.02 mg/ml) or "high" (0.04 mg/ml) doses of nicotine (added to drinking water), using [14C]bicarbonate as the label. It was found that the turnover of proteins in various subcellular fractions consisted of two distinct components turning over at a "fast" or a "slow" rate and having relatively short or long half-lives, respectively. Thus in control animals the half-lives of the protein components turning over at a fast rate ranged from 1.31 to 3.61 days whereas for those turning over at a slow rate the half-lives ranged from 8.56 to 24.28 days. Treatment with low doses of nicotine resulted in a more rapid turnover of nuclear fast turning over component with a concomitant decreased turnover of homogenate, cytosol, mitochondrial, and microsomal proteins; in the synaptosomal membranes this component disappeared altogether. The half-lives of the slow turning over components decreased in general from 14.3 to 33.3% with the exception of the nuclear proteins, where the half-live increased by 71.1%. Turnover of microsomal proteins was not affected. When the animals were given a high dose of nicotine, the turnover of fast components became even more rapid for nuclear, myelin, and microsomal proteins with a decrease in half-life from 26.6 to 32.3%. By contrast, half-lives of synaptosomal and mitochondrial proteins increased by 16.1-89.3%. These changes were not reflected in the turnover rate of whole homogenate proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The effect of ethanol consumption (27% of energy intake) for 21 days, on protein synthesis and degradation in subcellular fractions of rat brain was investigated using the Na2(14)CO3 labelling procedure. Ethanol administration elicited an increase in the rate of protein synthesis in all fractions which was accompanied by an increase of similar magnitude in the rate of breakdown. These results imply that whilst ethanol increases the rate of protein turnover it is without effect on the rate of protein accretion.

Rates of amino acid incorporation into particulate proteins in vivo and in slices of young and adult brain

Incorporation of a flooding dose of valine into brain proteins was measured in vivo and in brain slices, according to subcellular fraction and to solubility. In particulate fractions in vivo in the immature brain, soluble cytosol proteins had the highest synthesis rates (2.0% per hr), followed by microsomes (1.7%), mitochondria, nuclei (1.4%), and synaptosomes (0.85%). In adult, the rates were about half of those of immature animals except for microsomes, which were the same. In young, rates of incorporation into particles in brain slices were about 80% of in vivo rates, and adult rates were 10-20% of the in vivo rates. When brain proteins were fractionated into four groups--isotonic soluble, hypotonic soluble, Triton X-100 soluble, and the remainder solubilized in 0.2% SDS--in both young and adult brain in vivo, the isotonic fraction had the highest rate of protein synthesis, followed by the Triton-soluble fraction, the SDS-soluble fraction, and the hypotonic fraction. Also, with this method of separation in vitro rates for immature brain were 10-20% lower than in vivo rates, while in vitro adult rates were 80-90% lower. The results indicate that the most proteins are synthesized at a higher rate in the immature brain, and that the difference between in vivo and brain slice protein synthesis is not as great in the immature as in the mature brain in all the fractions examined.

Temporal profiles of proteins responsive to transient ischemia

The responses of long and short half-lived proteins to ischemia were measured in rat brain during 6 days of recovery from 30 min of transient forebrain ischemia produced by four-vessel occlusion. At the end of the ischemic interval, the neocortical activities of four vulnerable enzymes [ornithine (ODC) and S-adenosylmethionine (SAMDC) decarboxylases, and RNA polymerases I and II] were unchanged, but within 30 min of reperfusion, their activities dropped by 25-50%. The loss of substance P in the striatum and substantia nigra was slower, reaching about 50% by 12 h. On the other hand, the activities of 5 long half-lived enzymes did not change in the neocortex at 5 and 15 h of reperfusion and regional protein concentrations were essentially unaffected over 6 days survival. The rate and extent of normalization of the amounts or activities of the vulnerable proteins varied. RNA polymerase II and ODC activities were restored within 4 h, and ODC showed a biphasic increase in activity, with peaks at 10 h and 2-3 days. RNA polymerase I and SAMDC activities were restored by 18 h and 5 days, respectively, whereas substance P concentrations did not completely recover, even at 6-15 days. The greater the regional reduction of blood flow during ischemia, the larger the net change (gain or loss) of SAMDC or ODC activity and the longer the time required to normalize the activities of these enzymes. The average rate of proteolysis, assessed by measuring the rate of clearance of 14C from protein prelabeled with [14C]bicarbonate, was abnormal during the first 2 days of reperfusion. Postischemic changes in both protein synthesis and degradation could affect the amounts of some of the proteins responsive to transient ischemia.

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.