Biochemistry of Amino Acids - A. Majster 1961

Intermediary Metabolism of Amino Acids
Proline
Proline Metabolism in Mammals

The metabolic interrelationships between Proline, glutamic acid, and Ornithine in mammalian Tissues are similar to those found in microorganisms. The first indications of these relationships came from nutritional studies (p. 123); direct proof was provided by isotope tracer experiments. For example, when rats are fed labeled glutamic acid, the isotopic label is recovered in Proline and Arginine [355, 356]. Conversely, feeding N¹⁵-labeled proline results in the incorporation of the isotope into glutamic acid, arginine, and ornithine [357]. The conversion of deuterium-labeled ornithine into proline and glutamic acid has also been established [358]. Experiments with Liver and Kidney slices demonstrated the conversion of proline into glutamic acid [359, 360].

The possibility of several intermediates being formed in these reactions has been investigated, including a-keto-δ-aminovaleric acid (or its cyclization product, ∆¹-pyrroline-2-carboxylic acid), glutamic acid y-semialdehyde (∆¹-pyrroline-5-carboxylic acid), 2-pyrrolidone-5-carboxylic acid, and a-amino-δ-hydroxyvaleric acid. The results of experiments with E. coli and kidney slices indicate that a-amino-δ-hydroxyvaleric acid is likely not an intermediate. Although it is utilized by Neurospora, its utilization is preceded by a lag period before growth begins. Apparently, this involves the Conversion of the aforementioned acid into a more active intermediate, presumably glutamic acid y-semialdehyde [345, 360, 361]. There is no evidence supporting The intermediate formation of 2-pyrrolidone-5-carboxylic acid, although it is known that the L-isomer of this compound undergoes rapid METABOLISM in rabbits and can replace glutamic acid in the diet of certain microorganisms [362]. In contrast to the L-isomer, D-pyrrolidonecarboxylic acid is excreted in the urine following its ingestion, as well as after feeding D-glutamic acid [363, 364].

a-Keto-δ-aminovaleric acid, the product of the Oxidative Deamination of D-proline and D-ornithine by D-Amino Acid Oxidase, has been identified in the form of its 2,4-dinitrophenylhydrazone [365]. This hydrazone was also isolated following The oxidation of L-proline by L-Amino Acid Oxidase from rat kidney [366]. The free a-keto acid was obtained via the oxidative deamination of δ-N-carbobenzoxy-L-ornithine using L-amino acid oxidase from snake venom; upon removal of the protecting group, an equilibrium is established between a-keto-δ-aminovaleric acid and its intramolecular cyclization product, ∆¹-pyrroline-2-carboxylic acid. The latter compound apparently does not exist in equilibrium with ∆¹-pyrroline-5-carboxylic acid. The growth of certain proline-requiring E. coli mutants is supported by ∆¹-pyrroline-5-carboxylic acid, but cannot be sustained by ∆¹-pyrroline-2-carboxylic acid [292]. Currently, there is no conclusive Evidence indicating the formation of a-keto-δ-aminovaleric acid as an intermediate in The interconversions of proline, ornithine, and glutamic acid.

Rabbit kidney preparations oxidize L-proline to a compound that is likely glutamic acid y-semialdehyde [367, 368]. This compound was also obtained via Transamination between ornithine and a series of a-keto acids in rat liver preparations (p. 226). Taken together, the data suggest that glutamic acid y-semialdehyde serves as an intermediate in the proline–ornithine–glutamic acid pathway, although this has not yet been definitively proven.

Following the administration of N¹⁵-Glycine to rats, equivalent concentrations of the isotope were found in the a- and δ-amino groups of ornithine [369]. These results can be explained either by The transfer of the a-amino group of ornithine to the 5-position or by the derivation of both amino groups from a common nitrogen precursor. Stetten [370] fed rats DL-ornithine labeled with N¹⁵ at either the a- or δ-position for 9 days. Very little isotopic nitrogen from the 5-amino group of ornithine was incorporated into the a-amino groups of tissue protein arginine. This group participated in The formation of proline to a much lesser extent than the a-amino group, whereas the majority of the glutamic acid amino groups were derived from the 5-amino group of ornithine. It is possible that these results were influenced to some extent by The Use of racemic ornithine preparations in the experiments. However, the appearance of significant amounts of D-ornithine in the urine indicates that the bulk of the D-ornithine remained unmetabolized in the Organism. These findings are consistent with the initial conversion of ornithine into glutamic acid y-semialdehyde, with the transfer of the 5-amino group into a labile metabolic nitrogen pool that appears to be in equilibrium with glutamic acid. This explanation agrees with the observation that the a-amino group of ornithine serves as the primary precursor of proline nitrogen.

No data have yet been obtained to confirm or exclude the participation of a-acetyl or other a-acyl derivatives of glutamic acid or its y-semialdehyde in mammalian metabolism. The reduction of glutamic acid y-semialdehyde to proline in the presence of reduced diphosphopyridine nucleotide has been demonstrated in liver preparations [1093]. Ornithine transaminase activity has been detected both in mammalian tissue preparations and in microorganisms (p. 226). Overall, the evidence indicates the occurrence of the following transformations in the animal organism:

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