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Rate-limiting steps for protein synthesis in isolated rat liver cells. Role of aspartate availability.

ABSTRACT: Amino-oxyacetate (carboxymethoxylamine) was found to inhibit protein labelling in isolated liver cells. A similar degree of inhibition (about 70%) was observed of basal and substrate-stimulated rates of protein labelling, ruling out an action on the cellular energy state. Its effect does not seem to be related either to a perturbation of the reduction state of the NAD system or to rate changes in the gluconeogenic pathway. The following observations indicate that amino-oxyacetate inhibits protein labelling by limiting aspartate supply. Amino-oxyacetate was ineffective in a postmitochondrial supernatant under non-limiting amino acid supply conditions. The aspartate cellular content decreases in the presence of amino-oxyacetate, although most other amino acids tend to accumulate. L-Cycloserine was unable to decrease aspartate content and was ineffective in decreasing protein labelling. The inhibitory action of amino-oxyacetate was specifically reversed by incubating cells with amino acids that increase the cellular content of aspartate.

SUBMITTER: Perez-Sala D

PROVIDER: S-EPMC1147731 | biostudies-other | 1987 Mar

REPOSITORIES: biostudies-other

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Project description:1. Urate synthesis and other metabolic characteristics of isolated chicken hepatocytes were studied. 2. The distinction is made between immediate precursors of the purine ring (glycine, glutamine, aspartate, formyltetrahydrofolate, bicarbonate) and ultimate precursors from which the immediate precursors are formed in the liver. 3. In hepatocytes from well-fed chickens the rate of urate synthesis was not greatly increased by the addition of amino acids or NH(4)Cl, but in hepatocytes from 72h-starved chickens the rate was much increased when alanine or asparagine was added as the only substrate. Other amino acids, when added alone, did not affect the rate. The exceptional effect of alanine and asparagine is due to the ready formation of the immediate precursors. 4. Conditions are described under which glutamine, serine, glycine plus formate, ribose and glucose increased the rate of urate synthesis. 5. At 1mm-NH(4)Cl (a concentration not much higher than that of blood plasma) the rate of urate synthesis in the presence of lactate was increased, but higher concentrations inhibited urate synthesis in the presence of lactate or alanine; with alanine even 1mm-NH(4)Cl was inhibitory. 6. Glucose synthesis from lactate, alanine or dihydroxyacetone was also inhibited by 1mm-NH(4)Cl. 7. NH(4)Cl inhibition of urate and glucose synthesis was paralleled by an increased rate of glutamine synthesis. Thus in the presence of NH(4)Cl the gluconeogenic precursors are diverted from the pathway of gluconeogenesis to that of glutamate and glutamine synthesis. This implies that the synthesis of these amino acids is the primary process in the detoxication of ammonia in the avian liver. 8. Urate synthesis, like urea synthesis, can be looked on as a cyclic process with either phosphoribosyl pyrophosphate or ribose acting as the carrier on which the purine ring is assembled. 9. The energy requirements of urate synthesis depend on whether phosphoribosyl pyrophosphate is regenerated from IMP by pyrophosphorylase or by phosphorylation and pyrophosphorylation of ribose. It is 6 or 9 pyrophosphate bonds of ATP respectively.

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Rate-limiting steps for protein synthesis in isolated rat liver cells. Role of aspartate availability.

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