Amino acid transport radioactivity

Metabolic Transit. Free Amadori Compounds. It is well known that the synthetic Amadori compounds of the free amino acids are absorbed by the intestine and excreted unchanged in the urine (9,28,30). The transport is not active as observed with deoxyfructosyltryptophan (30) and c-deoxyfructosyllysine (40), and the level of absorption depends on the nature of the amino acid and on the conditions of ingestion. Nutritional assays and metabolic transit studies performed with radioactive Amadori compounds of tryptophan (12,30), leucine (12), and lysine (9,28,41) given orally or intravenously on normal or anti-biotics-treated animals have shown that the intestinal microflora can regenerate part of the amino acid. This can be absorbed subsequently at a very low level by the caecum or the large intestine and incorporated into the tissue proteins or utilized by the intestinal microflora. Barbiroli (13) showed also that some intestinal enzymes were able to liberate some amino acids from their Amadori compounds but to a very small... 

Pulse-labeling with radioactive amino acids can specifically label a cohort of newly made proteins in the ER. Alternatively, a temperature-sensitive mutant protein that is retained in the ER at the nonpermlsslve temperature will be released as a cohort for transport when cells are shifted to the permissive temperature. 

EXPERIMENTAL FIGURE 20-17 The rate of axonal transport in vivo can be determined by radiolabeling and gel electrophoresis. The cell bodies of neurons In the sciatic nerve are located In dorsal-root ganglia. Radioactive amino acids Injected Into these ganglia In experimental animals are Incorporated Into newly synthesized proteins, which are then transported down the axon to the synapse. Animals are sacrificed at various times after Injection and the dissected sciatic nerve Is cut Into small segments for analysis with the use of gel electrophoresis. The red, blue, and purple dots represent groups of proteins that are transported down the axon at different rates, red most rapidly, purple least rapidly. 

For many trypanosomatids it has been described that they are able to metabolize proline, which is abundantly present in the insect haemolymph, as the major nitrogen and carbon source. Proline uptake takes place via a carrier protein in the plasma membrane, but the nature of the transport is unclear. Active transport of proline into Leishmania spp. by means of a proton/proline symporter has been suggested using radioactive proline (52). The use of proline itself rather than a non-metabolizable analog, however, complicates the interpretation of the results since transport and metabolism cannot be separated and thus the intracellular proline concentration is easily overestimated. In chemostat-grown cells of L. donovani no evidence was found for any intracellular accumulation of this amino acid and it was suggested that proline was taken up by facilitated diffusion (74). 

The negative incorporations must be interpreted with caution, since solubility aspects and transport of the radioactive precursors to the site of biosynthesis may be extremely important 45, 47, 166). The last compound cited in Table III was incorporated into galantha-mine with no randomization (presumably via isovanillin), yet isovanillin was not utilized for alkaloid formation in either the Narcissus Deanna Durbin or in Nerine bowdenii 45, 47). Phenylalanine, an established precursor of the C-6—C-1 fragment of the alkaloids when injected into the leaves, flower stems, or bulbs, is not incorporated into alkaloids when introduced via root absorption from a hydroponic solution containing the radioactive amino acid 166). 

If radioactive amino acids are incorporated into mitochondrial proteins in vitro radioactivity is associated with mitoribosomes (Ashwell and Work, 1970). But this activity can be removed by puromycin treatment, suggesting that the radioactivity is the result of incomplete peptide chains on the mitoribosomes. There is no evidence that any mitochondrial ribosomal proteins are labeled in vitro although a clear-cut separation of the ribosomal proteins from animal cells has not been reported. Even with N. crassa no evidence was found that ribosomal proteins are coded for by mt DNA (Lizardi and Luck, 1972). From all these data it seems very likely that mitochondrial proteins are synthesized outside the mitochondria, under control of the nuclear genome, and are subsequently transported into the mitochondrial compartment. 

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