WHEN [14C]glucose is administered in oiuo, part is oxidized to CO,, part is stored as glycogen and part is used for the synthesis of amino acids and other compounds in the tissues (WINZLER et al., 1952;PROCHOROVA, 1954; ALLWEIS and MAGNES, 1958a,b; GEIGER, 1958;BUSCH et al., 1960). ROBERTS, FLEXNER and FLEXNER (1959) reported a relatively high rate of conversion of glucose carbon into protein in the liver and brain in the mouse. They found that the distribution of I4C in the different amino acids of the proteins was different in the two organs : there was a greater incorporation of I4C into alanine than into glutamic acid in the liver, whereas in brain tissue the converse was true.The object of the present work was to study the conversion of glucose-carbon into protein in the rat brain. Orientating experiments were first carried out to find out to what extent carbon from [14C]glucose is incorporated in the free amino acids and proteins of other organs besides the brain. The incorporation was studied for a period of up to 12 hr and the distribution of I4C was examined in the dicarboxylic, neutral and basic amino acid fractions. METHODSMale or female littermates (100-150 g) of a Wistar albino strain of rat were used. D-[UJ~C]-Glucose (410 pclmg) was obtained from the Radiochemical Centre, Amersham, England. The isotopic purity determined by the suppliers by dilution with glucose and conversion to the pentaacetate was 100 per cent 14C: analysis by paper chromatography gave 99 per cent purity both in n-butanolethanol-water and in phenol-water. Each animal received 0.2 ml of a solution of D-[u-' *C]glUCOSe (5 pc) containing 2 mg of added carrier glucose by subcutaneous injection. The animals were decapitated with a guillotine at intervals between 0 3 and 12 hr after injection of radioactive glucose. The brains and other organs were quickly removed, blotted on filter paper, frozen in liquid nitrogen and then kept at -20" until analysed. Blood was taken immediately after decapitation in a 25 ml beaker coated with Na-oxalate and 1 ml was pipetted with a similarly coated pipette into 5 ml of 10 per cent (w/v) trichloroacetic acid (TCA).Protein fractions. Brain, liver, kidney, spleen, heart, blood and skeletal muscle (gastrocnemius) were taken for analysis. The organs were weighed on a torsion balance, defrosted in 5 ml of ice-cold 10% TCA in a MSE-homogenizer tube (Measuring & Scientific Equipment Ltd., London), and then homogenized for 1 min at 0". The sediment obtained by centrifugation for 15 min at 1000 g was resuspended in 5 ml of TCA at 0" and the treatment was repeated with 4 consecutive 5 ml portions of TCA. The combined TCA-extracts were kept for analysis of acid-soluble amino acids.The residue after TCA extraction was treated with organic solvents to remove lipids as described by VRBA, FOLBERGROV~ and KANTBREK (1957). The tissue lipids were extracted consecutively with acetone, ethanol-ethyl ether (3 : I, v/v), chloroform-methanol (2 : 1 , v/v) and ether. The combined lipid extracts were evaporated to dryness ...

1. Rats were injected with [U-(14)C]glucose and after various intervals extracts of whole brain proteins (and in some cases proteins from liver, blood and heart) were prepared by high-speed centrifugation of homogenates in 0.9% sodium chloride or 0.5% sodium deoxycholate. 2. The extracts were subjected to gel filtration on columns of Sephadex G-200 equilibrated with 0.9% sodium chloride or 0.5% sodium deoxycholate. 3. Extracts prepared with both solvents displayed on gel filtration a continuous range of proteins of approximate molecular weights ranging from less than 2x10(4) to more than 8x10(5). 4. The relative amount of the large proteins (mol.wt.>8x10(5)) was conspicuously higher in brain and liver than in blood. 5. At 15min after the injection of [U-(14)C]glucose the smaller protein molecules (mol.wt.<2x10(4)) were significantly radioactive, whereas no (14)C could be detected in the larger (mol.wt.>2x10(4)) protein molecules. The labelling of all protein samples was similar within 4h after injection of [U-(14)C]glucose. Fractionation of brain proteins into distinctly different groups by the methods used in the present work yielded protein samples with a specific radioactivity comparable with that of total brain protein. 6. No evidence could be obtained by the methods used in the present and previous work to indicate the presence of a significant amount of ;metabolically inert protein' in the brain. 7. It is concluded that: (a) most or all of the brain proteins are in a dynamic state of equilibrium between continuous catabolism and anabolism; (b) the continuous conversion of glucose into protein is an important part of the maintenance of this equilibrium and of the homoeostasis of brain proteins in vivo.

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