Axonal transport and turnover of proline- and leucine-labeled protein in the goldfish visual system.

The suitability of radioactively labeled proline as a marker of axonally transported protein in the goldfish visual system is further investigated and compared with another amino acid, leucine, in double-label experiments. Intraocularly injected proline is incorporated into protein in the eye 5 times more efficiently than is leucine, while local labeling of brain protein from precursor which has left the eye and entered the blood, (observed in the ipsilateral optic tectum) is fiveto eight-fold less from proline than from leucine. The difference is attributed to the superior transport of leucine, an essential amino acid, into the brain from the blood. Once in the brain, the apparent rates of incorporation of the two amino acids are similar. Prolineor leucine-labeled, axonally transported proteins have a longer apparent half-life in the brain than do proteins labeled from intracranial injection of the precursors. By either route, proline-labeled proteins have a longer apparent half-life than leucine-labeled proteins. It is proposed that proline, released from protein breakdown is reutilized to a greater extent than is leucine. RADIOACTNE amino acids have previously been demonstrated to be useful in studying axonal flow in the visual system (TAYLOR & WE[?&, 1965). Following intraocular (10) injection into the eye of a goldfish, labeled amino acids are incorporated into retinal ganglion cell proteins, which are then transported along the optic nerve to the contralateral optic tectum (GRAFSTEIN, 1967). Since the visual pathways are completely crossed, radioactivity in the ipsilateral optic tectum is due to labeled amino acid which has escaped from the eye, entered the brain from the blood, and has been incorporated into protein locally. To assess the amount of axonally transported protein in the contralateral tectum, an amount equal to that found in the ipsilateral tectum is subtracted (MCEWEN & GRAFSTEIN, 1968). When leucine is used as the precursor, the ipsilateral labeling is more than 50 per cent of that seen on the contralateral side. Such