METABOLISM OF HIGHLY BASIC PROTEINS OF RAT BRAIN DURING POSTNATAL DEVELOPMENT

Abstract— (1) The encephalitogenic basic protein obtained from adult rat brain by treatment with 0·03 N-HCl was demonstrable in the brain on the 10th day after birth. It showed a marked increase in quantity during the phase of active myelination.
(2) The proteins extracted under similar conditions from 5-day old rat brain contained several highly basic proteins other than the encephalitogenic basic protein. These basic proteins, which were electrophoretically similar to highly basic proteins extracted similarly from adult rat liver, are histones.
(3) For metabolic studies the entire group of highly basic proteins in the acid extract was obtained after one-step adsorption of other proteins on DEAE-cellulose equilibrated at pH 9·8
(4) After injection of [¹⁴C]lysine the fractions containing highly basic proteins, water soluble non-basic proteins and other tissue proteins of the brain showed higher relative specific radioactivities during the period 1–10 days after birth than during later stages of postnatal development. The fraction containing proteolipid protein, another myelin protein, showed a low relative specific radioactivity throughout the whole period of postnatal development. The relative specific radioactivity of proteolipid protein was somewhat higher in young than in adult rat brain.     

DEVELOPMENT OF MITOCHONDRIAL PYRUVATE METABOLISM IN RAT BRAIN

The activities of a number of mitochondrial enzymes involved in the metabolism of pyruvate during development of the rat brain were investigated. The rates of decarboxylation of [1-¹⁴C]pyruvate to ¹⁴CO₂ via pyruvate dehydrogenase and the fixation of H¹⁴CO₃^? in the presence of pyruvate via pyruvate carboxylase by brain homogenates were very low in newborn rats. These rates increased markedly by about four-fold and 15-fold respectively during 10–35 postnatal days. The rates of the fixation of H¹⁴CO₃^? by cerebral homogenates were supported by the development of the activity of pyruvate carboxylase in rat brain. The activities of citrate synthase, aconitase, NAD-malate dehydrogenase, aspartate aminotransferase, alanine aminotransferase and phosphoenol-pyruvate carboxykinase were very low in the particulate fraction of the newborn rat brain. The activities of all these enzymes increased makedly by about three- to 10-fold during 10–35 days after birth. The activity of mitochondrial phosphoenolpyruvate carboxykinase from rat brain was not precipitated by an antibody prepared against rat liver cytosolic phosphoenolpyruvate carboxykinase suggesting that cerebral mitochondrial enzyme is immunologically different from that of the cytosolic form in hepatocytes. The significance of the development of the cerebral mitochondrial metabolism is discussed in relation to biochemical maturation of the brain.     

NUCLEAR RIBONUCLEASE ACTIVITIES OF RAT BRAIN DURING POSTNATAL DEVELOPMENT

Abstract— The activities of alkaline and acidic RNAses were determined in soluble and insoluble fractions from nuclei of brain hemispheres of rats, aged from 1 day to adult. The activities increased rapidly and reached a maximum, at 30 days, of about 10 times (alkaline RNAsel or 5 times (acidic RNAse) that seen at day 1.     

THE DESMOSTEROL REDUCTASE ACTIVITY OF RAT BRAIN DURING DEVELOPMENT1

Abstract— The reduction of desmosterol by cell-free preparations from developing rat brain was established and the age-dependent alterations in reductase activity were correlated with levels of desmosterol in brain. An increase in enzymic activity followed closely the sharp increase in levels of desmosterol that was observed at about 5 days of age and that reached a maximum at 8-11 days of postnatal age. Furthermore, the abrupt decrease in the desmosterol content of brain at 13-15 days of age was associated with a decrease in enzymic activity. We suggest that the enzyme may be substrate-induced and that this attribute may be of significance with respect to the ontogenesis of myelin. Cerebral desmosterol reductase exhibited a specific requirement for NADPH and was primarily a particulate enzyme.     

POLYAMINES and NUCLEIC ACID METABOLISM DURING DEVELOPMENT OF CHICK EMBRYO BRAIN

Abstract— Spermine and spermidine reach maximum concentrations in the chick embryo brain between the 12th and 14th day of incubation. Sucrose-density-gradient analysis of polyribosome distribution in the developing chick embryo brain, showed the presence of polyribosomal aggregates in the regions of 147 S and 206 S between the sixth and eighth day of incubation. After the 16th day of incubation the presence of heavier polyribosomal aggregates in the region of 259 S and 280 S was found. The injection of spermine or spermidine into the air space of embryos on the tenth day of incubation leads to a remarkable increase in the incorporation rate of [³H]formate into the ribosomes. Studies under similar experimental conditions, showed an increased radioactivity in the region of 147 S, 206 S, 259 S and 280 S in embryos injected with spermine or spermidine.     

MEMBRANE PROTEINS OF RAT BRAIN: COMPOSITIONAL CHANGES DURING POSTNATAL DEVELOPMENT

Abstract— Membrane fractions from forebrain of rat were isolated at ages ranging from 5 to 93 days. Among these fractions were total membranes, three fractions isolated by density gradient centrifugation, and three subfractions which consisted of purified myelin and of two supernatant fractions. All membrane fractions showed an increase in protein content during the first postnatal month; however, only the myelin fraction and one of its supernatant fractions showed a prolonged accumulation. Myelin protein increased continually from 0.17 mg/g brain at 15 days to 8.3 mg/g brain at 93 days.
All fractions were analysed for protein composition by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Characteristic changes in protein composition were noted during postnatal development, most of which were pronounced up to the age of 20 days. Among others was a decrease in histones as compared to other proteins, with a concomitant shift in preponderance from the slow- to the fast-migrating histone band. In parallel, other proteins of high molecular weight became more prominent. No myelin could be isolated at 5 and 10 days. The deposition of myelin proteins was parallelled by the appearance of the Wolfgram protein which points to a close correlation of the Wolfgram protein to the process of myelination.     

METABOLISM OF BRAIN DURING SLEEP AND WAKEFULNESS

Abstract— The levels in brain of lactate, pyruvate, creatine phosphate, ATP, ADP and AMP were examined in sleeping and waking adult rats. The animals were monitored electrophysiologically and the biochemical measurements were made after approx. 25 min of sleep or wakefulness. The previous treatment of the animals had a marked effect on the levels of brain metabolites during sleep. In animals not acclimatized to the observation chamber, brain levels of lactate and pyruvate rose during sleep above those in the waking state: creatine phosphate and ATP were depressed somewhat. When the animals were acclimatized by being placed in the observation chamber for at least 2 h on four or more consecutive days prior to the experiment, sleep was accompanied by a depression of brain levels of lactate and pyruvate and slight elevations of brain levels of creatine phosphate and ATP. No significant differences in the EEG recordings were noted between the sleeping rats of the acclimatized and non-acclimatized groups. These observations on the effect of acclimatization on brain metabolism during sleep may have clinical relevance in man.     

SYNTHESIS AND METABOLISM OF l-KYNURENINE IN RAT BRAIN

Abstract— A method for the quantitative analysis of femtomole amounts of kynurenine (along with tryptophan, 3-hydroxykynurenine and kynuramine) in rat brain using high pressure liquid chroma-tography and electron-capture GLC is described. Endogenous concentrations of these substances in rat brain regions were measured, and their formation after the injection of radioactive tryptophan or kynurenine was determined. Kynurenine was formed from tryptophan in brain and was also taken up from the periphery. Extracerebral kynurenine was calculated to account for 60% of the cerebral pool of kynurenine. The cerebral rates of synthesis of kynurenine and 3-hydroxykynurenine were 0.29 and 0.17nmol/g/h. The turnover rate of kynurenine in the brain was 1.02 nmol/g/h measured from [¹⁴C]tryptophan or 1.14 nmol/g/h from [³H]kynurenine injected intraperitoneally. Kynuramine levels in different areas of the brain were similar to those of tryptamine. Following intraperitoneal injection of [¹⁴C]tryptophan, the presence of anthranilic, 3-hydroxyanthranilic, xanthurenic, kynurenic and quinaldic acids was demonstrated in the brain.     

THE RELATIVE SIGNIFICANCE OF CO2-FIXING ENZYMES IN THE METABOLISM OF RAT BRAIN

To evaluate the relative significance of CO₂-fixing enzymes in the metabolism of rat brain, the subcellular distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase, as well as the fixation of H¹⁴CO₃^? by the cytosol and the mitochondria was investigated. Pyruvate carboxylase and phosphoenol-pyruvate carboxykinase are mainly localized in the mitochondria whereas NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase are present in both the cytosol and the mitochondria. In the presence of pyruvate rat brain mitochondria fixed H¹⁴CO₃^? at a rate of about 170 nmol/g of tissue/min whereas these organelles fixed negligible amounts of H¹⁴CO₃^? in the presence of α-ketoglutarate or phosphoenolpyruvate. Rat brain cortex slices fixed H¹⁴CO₃^? at a rate of about 7 nmol/g of tissue/min and it was increased by two-fold when pyruvate was added to the incubation medium. The carboxylation of α-ketoglutarate and pyruvate by the reversal of the cytosolic NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase respectively was very low as compared to that by pyruvate carboxylase. The rate of carboxylation reaction of both NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase was only about 1/10th of that of decarboxylation reaction of the same enzyme. It is suggested that under physiological conditions these two enzymes do not play a significant role in CO₂-fixation in the brain. In rat brain cytosol, citrate is largely metabolized to α-ketoglutarate by a sequential action of aconitate hydratase and NADP-isocitrate dehydrogenase. The operation of the citrate-cleavage pathway in rat brain cytosol is demonstrated. The data show that among four CO₂-fixing enzymes, pyruvate carboxylase, an anaplerotic enzyme, plays the major role in CO₂-fixation in the brain.     

NUCLEOTIDE METABOLISM IN RAT BRAIN

Abstract— The uptake, the conversion to nucleotides, and their incorporation into RNA for labelled glycine, aspartate, the free bases and nucleosides of purines and pyrimidines were investigated with cortical slices of rat cerebrum. At the end of a 1-hr incubation time the slice-to-medium ratio of the radioactivities for labelled aspartate, glycine, adenine and adenosine were 34, 26, 20 and 5, respectively, while the slice-to-medium ratios for hypoxanthine, inosine, guanine, guanosine, xanthine, orotate, cytidine, cytosine, uridine, and uracil ranged from 1.3:1 to 2:1. Over 99 per cent of the total radioactivity taken up by the cortical slices was present in the TCA supernatant and 86, 82, 65, 50, 34, 23, 20 and 1.6 per cent of this radioactivity was in the form of nucleotides at the end of a 1-hr incubation with labelled adenine, adenosine, hypoxanthine, inosine, uridine, orotate, cytidine, and glycine, respectively. The incorporation of various radioactive precursors into RNA of cortical slices suggests that nucleotides originating from either de novo synthesis or preformed purine derivatives enter the same nucleotide pool utilized for RNA synthesis. The supernatant fraction from homogenized cerebrum was investigated for the presence of various anabolic and catabolic enzymes associated with nucleotide metabolism. These results were correlated with the data from the RNA incorporation studies, and a possible role for AMP: pyrophosphate phosphoribosyltransferase (adenine phosphoribosyltransferase, I.U.B. 2.4.2.7) to achieve intercellular transfer of AMP is discussed.     

RNA METABOLISM IN ISOLATED MOUSE BRAIN NUCLEI DURING EARLY POSTNATAL DEVELOPMENT

—RNA metabolism in isolated brain nuclei has been shown to be dramatically altered during early postnatal brain development. The present study involved an examination of the RNA products synthesized by nuclei at various stages of postnatal neural maturation. In all cases, the majority of the RNA appeared to be heterodisperse, non-ribosomal and non-tRNA in nature. In comparison to the RNA isolated from nuclei of neonatal tissue, the RNA from nuclei of 12-day and 30-day-old mouse brain was found to be of smaller molecular weight. Despite the heterodisperse nature of these RNA molecules, the addition of α-amanitin did not completely inhibit nuclear synthesis. An investigation of RNA synthesis in isolated neuronal and glial cell nuclei revealed that nucleic acid metabolism in these respective cell populations had different and distinct developmental patterns. Preparations enriched with glial cell nuclei were found to be most active at birth and then decreased in activity (3–4-fold) during neural maturation. On the other hand, the rate of RNA synthesis in fractions enriched in neuronal cell nuclei was observed to increase dramatically in activity (4–5-fold) until 14 days of age. From 14 days of age until adulthood, RNA synthetic activity remained essentially the same.     

METHYLATION OF MYELIN BASIC PROTEIN BY ENZYMES FROM RAT BRAIN

Abstract— In rat brain Methylase l activity ( S -adenosyl- l -methionine: protein-arginine methyl-transferase) is found predominantly in the cytoplasmic fraction, and it appears that several enzymes contribute to this activity. No evidence for the existence of two enzymes specific for the methylation of histone and myelin basic protein was found. The specific activity of Methylase I did not increase at the period of rapid synthesis of myelin basic proteins. Methylase I activity was strongly inhibited by S -adenosyl- l -homocysteine.     

SUBCELLULAR DISTRIBUTION OF 5-HYDROXYTRYPTAMINE IN RAT BRAIN DURING DEVELOPMENT: EFFECT OF DRUGS AND FASTING

Abstract— Distribution of brain 5-HT content between the high-speed supernatant and particulate fractions under normal and experimental conditions was studied in postnatal and adult rats. In adult and 35-day-old rats the 5-HT content of the supernatant fraction was about 25% of that of the total homogenate and significantly higher than that in 1, 7 and 21-day-old rats. In 1-day-old rats fasting caused an increase of 100% in the turnover, 50% in the content and no alteration in the subcellular distribution of brain 5-HT, which suggests that under normal conditions 5-HT stores may be filled near to capacity. After 5-hydroxytryptophan administration, the 5-HT content of the adult rat brain increased 3-fold and that of the supernatant fraction to 35% of 5-HT content of the total homogenate. In postnatal rats, the brain 5-HT content rose to an adult level and the supernatant 5-HT percentage to a markedly higher than adult level, probably because of the known higher than adult 5-hydroxytryptophan decarboxylase activity of brain capillaries. Administration of tranylcypromine to adult rats caused a 2.6-fold increase of brain 5-HT content and a slight increase of the supernatant 5-HT percentage. At various times after the administration of the MAO inhibitors (tranylcypromine or pargyline) and fasting to the 1-day-old rats, brain 5-HT content increased 4, 5 and 7-fold, respectively, and the supernatant 5-HT rose consistently and, as in the adult, to about 30% of the 5-HT content of the total homogenate. After pargyline following reserpine pretreatment, the 5-HT content of the adult and 1-day-old rat brain increased to 2–3 times the control level and that of the supernatant fraction to about 40% of the 5-HT content of the total homogenate. The adult values for 5-HT in the particulate fraction of the 1-day-old rats after the drug treatments are in sharp contradiction to the low endogenous 5-HT content and known lack of nerve terminals and synaptic vesicles in their brains, and suggest that after MAO inhibition brain 5-HT neurons may bind the amine by some other mechanism than the Mg²⁺-ATP-dependent, reserpine-sensitive granular storage.     

COMPOSITION OF MOUSE BRAIN MYELIN DURING DEVELOPMENT

Myelin was isolated from the brains of mice at ages of 14, 24, 41, 44, 47, and 182 days and the contents of lipid phosphorus, cholesterol, lipid galactose, alkenyl groups, ethanolamine phosphoglycerides, choline phosphoglycerides, sphingomyelin, and serine and inositol phosphoglycerides were determined. Significant differences in the composition relative to total lipid phosphorus were found in the myelin. At 14 days of age, the myelin had lower relative amounts of cholesterol, galactolipids, alkenyl groups, and ethanolamine phosphoglycerides and a higher relative amount of choline phosphoglycerides.     

EFFECTS OF HYPOPHYSECTOMY ON RNA METABOLISM IN RAT BRAIN STEM

Abstract— Ribosomal aggregates were isolated from rat brain stem and characterized as polysomes by sedimentation analysis and by their sensitivity to RNase and EDTA treatment.
Three weeks following hypophysectomy there was a significant decrease in the content of large polysomes in the rat brain stem. The incorporation of radioactive uridine into RNA was studied using a double-labelling technique with [³H]- and [¹⁴C]uridine and labelling periods of 70 and 180 min. It was found that after hypophysectomy the incorporation of radioactive uridine into total, nuclear and cytoplasmic RNA and in polysomes was decreased after 70 and 180 min. Information on the nature of the rapidly-labelled RNA in the various subcellular fractions was obtained by sucrose gradient sedimentation analysis.
After 70 min of labelling the nucleus contained heterogeneous RNA with a considerable fraction of RNA sedimenting faster than 28 S. In the cytoplasmic fraction heterogeneous 4 to 30 S RNA was found, presumably associated with RNP particles, whereas after 180 min the polyribosomal aggregates were also labelled.
The present results indicate a profound effect of hypophysectomy on the metabolism of all species of brain RNA investigated.     

METABOLISM OF MALONIC ACID IN RAT BRAIN AFTER INTRACEREBRAL INJECTION

Labeled malonic acid ([1-¹⁴C] and [2-¹⁴C]) was injected into the left cerebral hemisphere of anesthetized adult rats in order to determine the metabolic fate of this dicarboxylic acid in central nervous tissue. The animals were allowed to survive for 2, 5, 10. 15 or 30min. Blood was sampled from the torcular during the experimental period and labeled metabolites were extracted from the brain after intracardiac perfusion. There was a very rapid efflux of unreacted malonate in the cerebral venous blood. Labeled CO₂ was recovered from the venous blood and the respired air after the injection of [1-¹⁴C]malonate but not after [2-¹⁴C]malonate. The tissue extracts prepared from the brain showed only minimal labeling of fatty acids and sterols. Much higher radioactivity was present in glutamate, glutamine, aspartate, and GABA. The relative specific activities (RSA) of glutamine never rose above 1.00. Aspartate was labeled very rapidly and revealed evidence of ¹⁴CO₂ fixation in addition to labeling through the Krebs cycle. GABA revealed higher RSA after [1-¹⁴C]malonate than after [2-¹⁴C]malonate. Sequential degradations of glutamate and aspartate proved that labeling of these amino acids occurred from [1-¹⁴C] acetyl-CoA and [2-¹⁴C] acetyl-CoA, respectively, via the Krebs cycle. Malonate activation and malonyl-CoA decarboxylation in vivo were similar to experiments with isolated mitochondria. However, labeled malonate was not incorporated into the amino acids of free mitochondria. The results were compared to data obtained after intracerebral injection of [1-¹⁴C]acetate and [2-¹⁴C]acetate.