Amino Acid Recycling in Relation to Protein Turnover

Abstract

Methods of measuring amino add recycling in Lemna minor are argued elsewhere (4) that the kinetic data of these experiments have been misinterpreted, because the kinetic treatment appropriate for a "label" experiment was applied to a "chase" experiment, so that the analysis not only gives the wrong rate, but the inverse of the true rate.Measurements of protein degradation based on tritium labeling of protein have shown that when Lemna minor is deprived of nitrogen its growth rate is greatly reduced and protein degradation is increased (10). This suggests an alternative strategy which the plant might employ to adapt to the deprivation of nitrate. The plant could change its enzyme complement by rapidly synthesizing new proteins from amino acids formed by degradation of old proteins.The reutilization of amino acids -amino acid recycling -is one of the main contributing factors to underestimating the rate of degradation of proteins. Protein half-lives are most frequently measured by labeling cells with radioactive amino acids and then following the decrease in trichloroacetic acid-precipitable radioactivity. The error in this method, which is produced by amino acid recycling, is well documented (13) and attempts have been made to evaluate the extent of amino acid recycling in animal tissues (3,16). Koch (14) concluded that "no matter how an isotope experiment is conducted the true half-life cannot be calculated without information additional to the time course of activity in the component under study."We have adapted the technique of density labeling pioneered by Varner (18)

Uptake of Leucine and Tyrosine and Their Intracellular Pools in Chlorella fusca var. vacuolata

“…Trapping of radioactive amino acids released from prelabelled protein is therefore not a suitable method for measuring overall rates of protein degradation (turnover) in this organism. The results of Halvorson (1958 a, b) for Saccharomyces cerevisiae and of Davies & Humphrey (1978) for Lemna minor and Schizosaccharomyces pombe suggest that this restriction is common in eukaryotic organisms.…”

Amino Acid Metabolism of Lemna minor L.

“…Certain amino acids (notably proline, falanine, and methionine) became increasingly larger percentages of the total free amino acid pool as the concentration of chlorsulfuron was increased (Table II). In contrast, the percentage of branched-chain amino acids (valine, leucine, and isoleucine) decreased relative to the total free amino acid pool (Table II) (5).…”

“…Vol. 84, 1987 The failure to achieve complete depletion of the branchedchain amino acids in chlorsulfuron treated L. minor appears to be the consequence of amino acid recycling associated with protein turnover (5,15). In the search for selective inhibitors of amino acid biosynthesis as targets of herbicide action (9), it should be recognized that total depletion of the target protein amino acid in the free pool may not be possible if protein turnover rates are appreciable in the test organism.…”

Catalase Degradation in Sunflower Cotyledons during Peroxisome Transition from Glyoxysomal to Leaf Peroxisomal Function

“…Density Labeling Experiments. Determinations of degradation constants of protein by measuring the loss of radioactivity from labeled protein can be affected by recycling of labeled compounds from degraded protein into protein synthesis (5,6). Recycling leads to an underestimation of degradation constants.…”