Effects of Chloramphenicol on the Uptake and Incorporation of Amino-acids by Carrot Root Tissue

Abstract

The uptake of uniformly labelled L-glutamic acid-¹⁴C, glycine-¹⁴C, and L-proline-¹⁴C by carrot root tissue is inhibited by chloramphenicol at a concentration of 2 g./l. L-glutamic acid absorption is affected to a much greater extent than is the uptake of the other two amino-acids. It is shown that the ¹⁴C from the labelled amino-acids is incorporated into protein of the carrot slices, and that the amount of incorporation from glycine and L-proline is about six times as large as that from L-glutamic acid under comparable conditions. Almost all the ¹⁴C detected in the protein after feeding labelled L-proline or glycine remains in the amino-acid supplied; in the case of L-glutamic acid some of the label is transferred to other amino-acid residues, but most of it remains in glutamic acid. Chloramphenicol is found to inhibit ¹⁴C-incorporation from L-proline and glycine in short-term, but not in long-term, experiments whereas incorporation from L-glutamic acid is inhibited in both long- and short-term experiments. Chloramphenicol also inhibits incorporation of ¹⁴C from L-glutamic acid into proteins of preparations of isolated subcellular particles from carrot root tissue. Under some conditions, ¹⁴C incorporation into protein proceeds for some time after the slices have been removed from the radioactive solution. Such incorporation is not inhibited significantly by chloramphenicol when ¹⁴C-labelled L-proline is supplied, but it is reduced by about 50 per cent. when glycine and L-glutamic acid are used. It is shown that chloramphenicol inhibits net protein synthesis in carrot root slices suspended in aerated solutions. It is concluded that amino-acid incorporation into the protein of carrot slices proceeds by at least two mechanisms, one of which is related to protein synthesis and is sensitive to chloramphenicol and the other is unrelated to protein synthesis and insensitive to the drug. Most of the L-proline and glycine incorporation into carrot root tissue protein seems to occur by the chloramphenicol-insensitive mechanism while much of the L-glutamic acid-¹⁴C incorporation seems to take place by the chloramphenicol-sensitive one.