Conversion of glutamate to ornithine and proline: pyrroline-5-carboxylate, a possible modulator of arginine requirements.

The many enzyme reactions involved in the interconversion of glutamate, ornithine, proline and arginine are interrelated to one another by the metabolite pyrroline-5-carboxylate. This chemical is synthesized de novo from glutamate by the intestinal mucosa and to a lesser degree by the thymus. Pyrroline-5-carboxylate can be metabolized to ornithine (and arginine), proline and glutamate. Several futile cycles could exist because of the common utilization of pyrroline-5-carboxylate for these pathways. This does not appear to occur because of the tissue distribution of these enzymes, the various isozymes of the enzymes that may have different regulatory or kinetic properties and amino acid transport systems that differ from tissue to tissue.