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

1. Rats were injected with [U-(14)C]glucose and the content of (14)C in proteins and lipids of the cerebral P(1) (;nuclear'), P(2) (;mitochondrial'), P(3) (;microsomal') and high-speed supernatant fractions was measured 7, 22 and 93hr. after injection of labelled glucose. 2. The crude brain mitochondrial fractions (P(2)) were subfractionated on continuous sucrose gradients (0.32-1.8m-sucrose) and the (14)C content of the proteins and lipids of about 20 subfractions was measured. 3. About 40-50% of the (14)C assimilated by brain proteins was found in the P(2) (;mitochondrial') fraction. About 68-70% of the (14)C assimilated by brain lipids was also recovered from the lipids of the P(2) fraction. 4. Between 22 and 93hr. after injection of [U-(14)C]glucose both the amount of (14)C in the protein of the P(2) (;mitochondrial') fraction and the specific activity of this protein increased. The specific activity of the protein of all other particulate fractions (P(1), P(2) and P(3)) and subfractions (obtained from sucrose-density-gradient subfractionation of fraction P(2)) when related to the specific activity of the high-speed supernatant protein also increased during 93hr. after injection of [U-(14)C]glucose. The amount of (14)C in the protein of the high-speed supernatant and the specific activity of this protein decreased during the same period. 5. The distribution of (14)C in the lipids of all subcellular particulate fractions remained unchanged during the period 22-93hr. after injection of [U-(14)C]glucose. 6. It was concluded that a diffusion occurs of some supernatant proteins into subcellular particulate matter of the cerebrum and no significant preference for any subcellular 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. 7. 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.