Liver amino acid equilibria

The isolation of an SCP protein from rat liver homogenates has also been reported (S2). This protein has been found to be heat-labile, to be detectable only in the liver, and to have a molecular weight of approximately 50,000 daltons by gel filtration (S2) and 28,000 daltons by sedimentation equilibrium (S3). Although the functional properties of the heat-labile SCP (SI) are similar to the heat-stable SCP (R2, R3), these proteins appear to be different. According to Scallen et al. (S3), their SCP preparation resembles chemically serum LDL this based on the similarity in amino acid composition between these two proteins. In the... 

Each tissue, including the bloodstream, has a free amino acid pool. This amounts to a total of about 100 g. By far the largest fraction, 50-80%, is located in muscle. Kidney accounts for about 4%, liver for 10%, and the bloodstream another 4%. Glutamine and glutamate are major components of such pools. Free amino acid pools are in equilibrium with tissue protein. Tissue proteins are in a constant state of turnover, that is, biosynthesis and degradation from and to free amino acids. Only plasma proteins, which are largely synthesized in the liver, are not in equilibrium with the plasma free amino acid pool. 

The MW of the NADP-specific GDH of Neurospora has been shown to be 280,000-296,000 by sedimentation equilibrium analysis (Wootten, 1973) while the amino acid sequence (Wootten et al., 1974) shows that the polypeptides possess 50 residues less than those of the bovine liver enzyme, and that each subunit has a MW of 48,438 showing that this enzyme also is hexameric. The NADP-specific enzymes of Baker s yeast, E. coli, and the enzyme of dual coenzyme specificity from Mycoplasma laidlawii have been shown to have molecular weights similar to the Neurospora NADP enzyme, while SDS electrophoresis has shown their subunits are also of a similar size (Smith et al., 1975). 

Although the equilibrium of this reaction is very much in favour of glutamate formation, in the cell the rapid removal of the 2-oxoglutarate and NAD(P)H allows the enzyme to function efficiently in the direction of glutamate deamination. Liver glutamate dehydrogenase is a very active enzyme, and the reaction is not rate-limiting for amino acid deamination. 

Catabolism of methionine is probably initiated by its conversion to jS-adenosylmethionine (14 ). This is the form of the amino acid that participates in transmethylation reactions. After loss of the methyl group iS-adenosylhomocysteine remains. De la Haba and Cantoni (143) showed that an enzyme occurs in liver which dissociates this compoimd into adenosine and homocysteine, although the equilibrium is greatly in favor of the ssmthesis of [Pg.112]

Second, the over-all rate of protein synthesis is not uniform- It varies from one type of tissue to another, and from one tjrpe of organism to another. Also, of course, since the amounts of different amino acids in proteins are not the same, the apparent rate of synthesis will depend on the amino acid whose incorporation is being studied. There again, the differential between several amino acids will vary from tissue to tissue, and in a way not entirely accountable by the differences in amino acid composition (69). The apparent rate of incorporation will also vary with the size of the intracellular amino acid pools, which may be quite large (and variable) and cause a very substantial dilution of the specific activity of the injected amino acid. Furthermore, as was shown by Loftfield and Harris (70) for liver, the intracellular amino acids are not in equilibrium with the circulating amino acids, since the specific activity of intracellular leucine never exceeded 40 % of that injected, even after 80 min. The specific activity of the amino acid at the site of synthesis may vary even more, but this is difficult to determine exxierimentally. There are also indications that there are different kinds of pools, some of which can exchange more... 

The conversion of ornithine to A -pyrroline -carboxylate is better understood, since the transamination reaction by which this occurs has been studied in some detail with Neurospora 121) and liver extracts 122). The equilibrium of the transaminase reaction with a-ketoglutaric acid as amino group acceptor favors A -pyrroline-5-carboxylate formation about 90% of the ornithine is decomposed. 

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