Movement of (U- 14 C)glucose carbon into and subsequent release from lipids and high-molecular-weight constituents of rat brain, liver, and heart in vivo.

Abstract

After subcutaneous injection of [U-14C]glucose into rats the amount of 14C incorporated in vivo into proteins was always higher than into lipids in brain, liver, and heart. The specific radioactivity of brain proteins was higher than those of liver and heart. Blood-brain comparisons show that protein carbon is derived continuously from glucose in the brain in situ and not as a result of deposition of amino acids or proteins from the circulation. Seventy-two percent of 14C in purified brain protein fractions was found in the amino acids of the hydrolysates of these fractions, mainly in alanine, glutamic, and aspartic acids. Maximum labelling was reached about 4 h after injection of [U-14C]glucose. Elimination of 14C from three classes of brain proteins (high-speed supernatant, particulate deoxycholate extractable, and residual) followed a biphasic time-course. The extent of labelling of, and the rate of elimination of 14C from, the three classes of rat brain proteins were very similar. The fate of 14C in the other investigated tissue fractions of brain, liver, and heart was compared with the fate of 14C in brain proteins.The results lend further support to the previously published suggestion that: (a) brain does not contain appreciable amounts of metabolically inert proteins or of proteins with turnover rates significantly higher than the mean for the bulk of brain proteins; (b) glucose carbon participates at a different rate and to a different extent in the metabolism of high-molecular-weight constituents of brain as compared to liver, heart, and plasma proteins; (c) the continuous conversion of glucose carbon into protein is an important part of the maintenance of the homeostasis of tissue proteins in vivo.