AXOPLASMIC INCORPORATION OF AMINO-ACIDS IN A MYELINATED FIBER EXCEEDS THAT OF ITS SOMA - A AUTORADIOGRAPHIC STUDY
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1983
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The axoplasmic incorporation of amino acids was studied in the giant Mauthner axon [of Carassius auratus] to determine its magnitude, to estimate the half-life of the resulting material, and to establish whether or not it undergoes transport. The incorporation of lableled leucine, lysine and proline was followed by radioautography. The tracers were injected away from the cell body, either in the floor of the IV ventricle or into the spinal cord. Radioautographic reaction on the Mauthner somata was barely discernible. At the site of injection, for survivals to 5.6 days, a peak of reaction was observed on the Mauthner axoplasm, with a sharp and symmetric decrement in both central and distal directions; the intensity of the peak decreased with survival (14% at 5.6 days). In intact fibers, the intensity of the reaction was 1.4% compared with that over the Nissl substance of neighboring somata; by contrast, in severed fibers whose axoplasm had been directly exposed to the tracers the reaction rose to 4.1%. At early times, the intensity of the response of the axoplasm as a function of survival of the fish did not have a lag time. In high-resolution radioautograms, most of the grains overlay the ground axoplasm. Cycloheximid depressed the reaction over the fiber open to the extracellular space by 55% whereas only by 20% over the intact fiber. Adsorption of free amino acids or their binding to tRNA were ruled out as a cause of the radioautographic response. The results indicated that incorporation of amino acids into macromolecules of the Mauthner fiber occurred locally, and its magnitude over the whole axoplasm was estimted to be one order of magnitude larger than that occurring in the soma. The bulk of the material resulting from this incorporation did nto undergo transport and survived some days. Evidently, the amino acids were incorporated by a ribosomal mechanism.