The effect of feeding with a tryptophan-free amino acid mixture on rat-liver polysomes and ribosomal ribonucleic acid

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Abstract
Fastlng rats were given complete and tryptophan-deficlent amino acid mixtures by stomach tube and were killed from 1 to 7 hr. later. The polysome profile In the livers of rats fed with the tryptophan-deficient mixture showed a shift In distribution such that the large aggregates were decreased and the small aggregates were increased, particularly dimers. This polysome shift was reversed when the complete amino acid mixture was given by stomach tube 2 hr. after administering the tryptophan-free amino acid mixture. After removal of liver polysomes by centrlfugatlon, some smaller rlbosomal aggregates (ollgo-somes) remaining in suspension were harvested by prolonged centrifuga-tlon of the supernatant fluid. A large increase in the dimer population of this fraction was observed in the rats receiving the incomplete mixture. When the polysome and oligosome fractions were incubated with cell sap, an energy-generating system and labelled amino acids DL-[1-C14 leuctne and L-[Me-C14] tryptophan were incorporated into the cell fractions in the ratio 4.5:1. Preparations of polysomes and oligosomes from rats fed with the tryptophan-free amino acid mixture showed a decreased amino acidincorporatingactivity compared with particulate preparations made from rats fed with the complete mixture. The yield of free ribosomes prepared from the unfractionated liver microsomes by treatment with isooctane was 40-50% greater in rats fed with the amino acid mixture deficient in tryptophan. A post-micro-somal fraction was prepared from cell sap and was shown to consist of ribosomal sub-units. When the animals were fed with the tryptophan-deficient mixture, there was an increase in content of this post-micro-somal fraction and in the ration 30s RNA: 19s RNA. Rats were also given [5-H3]orotic acid at the time of feeding with the amino acids. Lack of tryptophan in the mixture caused a decrease in the specific activity of both RNA fractions which affected the 30s RNA more extensively than the 19s RNA. These changes In the distribution and quantity of the cellular components engaged in protein synthesis are discussed in relation to RNA metabolism and amino acid-Incorporating activity of the liver cell and their response to feeding with the tryptophan-free amino acid mixture.