Nitrogen Economy and the Metabolism of Serine and Glycine in Reticulocytes of Rabbits

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Abstract
NH3 potentially available from the oxidation of amino acids by the [rabbit] reticulocyte is utilized for the new synthesis of serine via transamination reactions with hydroxypyruvate and phosphohydroxypyruvate. These compounds are derived from glucose, which furnishes the C skeleton of serine. Serine is mainly degraded via glycine, while glycine is mainly formed from serine. Serine synthesis is localized in the cytosol, while the reversible transformation of serine to glycine takes place in the mitochondria. A transfer of methylenetetrahydrofolate occurs in both directions across the mitochondrial membrane. A concentrative uptake of serine by the mitochondria was found, while glycine is transported slowly in both directions. On the basis of the data an overall balance is given of the quantitative relations between protein breakdown and Hb synthesis and for the new formation and utilization of serine and glycine. The new formation of serine may amount to about 1/10 of the glucose utilized by the reticulocyte and furnishes about 1/2 of the serine and glycine moieties required for the synthesis of heme and globin. The remainder is provided by the energy-dependent protein breakdown.