Figure 2 from Amino Acid recycling in relation to protein turnover. | Semantic Scholar (2025)

The results are discussed in relation to the differences in stability of the various enzymes relative to the overall protein turnover rate.

The results are discussed in relation to the differences in stability of the various enzymes relative to the overall protein turnover rate.

Analyses of the (15)N abundance of free amino acids suggest that protein turnover is the major source of these methionine sulfoximine induced amino acid accumulations, and the pools of valine, leucine, isoleucaine, proline, and threonine accumulated in response to the inhibitor are (14)N enriched and are not apparently derived from ( 15)N-labeled precursors.

It is suggested that this high rate of turnover results from the exposure of the culture to high levels of radioactivity for long periods, and it is concluded that neither method alone will yield an absolute value for the rate of protein turnover, but differences between classes of protein with different decay rates will be apparent.

Investigation of several enzymes indicates that all proteins are catabolised in response to 2H2O treatment and the implications of these results with regard to the interpretation of density-labelling experiments are discussed.

The first direct evidence for protein turnover in plants was provided independently by Hevesey et al. (1940), who reported the assimilation of [15N]H3 into leaf protein, despite a net loss of protein from leaves, who proposed a “PROTEIN CYCLE” according to which proteins undergo a cycle of synthesis and degradation.

The results suggest that glutamate, alanine, and asparagine are normally catabolized to ammonia by transaminase-dependent pathways rather than via dehydrogenase or amidohydrolase reactions.

Chlorsulfuron did not inhibit the methionine sulfoximine induced accumulations of valine, leucine, and isoleucines, supporting the idea that the accumulation of the branched-chain amino acids in methionines treated plants is the result of protein turnover rather than enhanced synthesis.

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There are at least 2 amino pools for leucine and for valine in the soybean hypocotyl, a small protein precursor pool and a large inactive pool, and the precursor pool was subject to expansion by supplying the amino acid externally at high concentrations.

To obtain satisfactory results it was necessary to improve the method of isopycnic centrifugation in CsCl gradients, and the advantages of using KBr rather than Cs Cl for the separation of density-labelled protein are discussed.

The specific measurement of 2-3H in amino acids largely overcomes problems due to compartmentation and isotope recycling and evidence to support this claim is presented.

Results suggest that 3H enters and leaves amino acids at the alpha-carbon atom, a conclusion supported by the observed labelling of glutamates.

"Heavy" labeling of Lemna with D(2)O and (15)NO(-) (3) has provided evidence for the conservation of ribosomal RNA in fast growing cultures and has also provided an alternative assessment of recycling.

Protein Synthesis and Turn-Over in Cultured Plant Tissue and the Fate of the Protein Breakdown Products: Sources of Carbon for Syntheses and the fate of the protein breakdown products is studied.

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