Assimilation of glucose carbon in subcellular rat brain particles in vivo and the problems of axoplasmic flow

Abstract

Rats were injected with [U-14C]ghicose and the content of 14C in proteins and lipids of the cerebral P1 ("nuclear"), P2 ("mitochondria!"), P3 ("microsomal") and high-speed supernatant fractions was measured, 7, 22 and 93 hr. after injection of labelled glucose. The crude brain mitochondrial fractions (P2) were subfractionated on continuous sucrose gradients (0.32-1.8M-sucrose) and the 14C content of the proteins and lipids of about 20 sub-fractions was measured. About 40-50% of the 14C assimilated bybrain proteins was found in the P2 ("mitochondria!") fraction. About 68-70% of the 14C assimilated by brain lipids was also recovered from the lipids of the P2 fraction. Between 22 and 93 hr. after injection of [U-14c]ghicose both the amount of 14C in the protein of the P2 ("mitochon-drial") fraction and the specific activity of this protein increased. The specific activity of the protein of all other particulate fractions (Pi, P2 and P3) and subtractions (obtained from sucrose-density-gradient subfractionation of fraction P2) when related to the specific activity of the high-speed supernatant protein also increased during 93 hr. after injection of [U-l4C]glucose. The amount of 14C in the protein of the high-speed supernatant and the specific activity of this protein decreased during the same period. The distribution of 14C in the lipids of all subcelhilar particulate fractions remained unchanged during the period 22-93 hr. after injection of [U-14C]glucose. It was concluded that a diffusion occurs of some supernatant proteins into subcellular particulate matter of the cerebrum and no significant preference for any subcelhilar particulate matter was observed. The lipids occur in the cerebrum mainly in a non-diffusible state, which is consistent with the view that they form almost entirely a part of the structure of the cerebrum. The data obtained do not lend further support to the concept of axoplasmic flow within the cerebrum or the concept of a one-directional flow of mitochondria or other subcellular particles within the cerebrum.