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 S 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 five‐ to 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. Proline‐ or 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.