Transfer of reducing equivalents into mitochondria by the interconversions of proline and delta 1-pyrroline-5-carboxylate.
Direct evidence is presented for a proline cycle using a cell-free experimental system which sequentially transfers 3H from [1-3H]glucose to NADP+ to delta 1-pyrroline-5-carboxylate and yields [3H]proline. The formation of [3H]proline depends on the presence of NADP, delta 1-pyrroline-5-carboxylate, and the enzymes glucose-6-phosphate dehydrogenase and delta 1-pyrroline-5-carboxylate reductase. The production of [3H]proline from unlabeled proline in the presence of mitochondria provides direct evidence for one complete turn of a proline cycle which transfers reducing equivalents produced by glucose oxidation in the pentose pathway into mitochondria. In this cycle, proline is oxidized to delta 1-pyrroline-5-carboxylate by mitochondrial proline oxidase. delta 1-pyrroline-5-carboxylate is released from mitochondria and is recycled back to proline by delta 1-pyrroline-5-carboxylate reductase with concomitant oxidation of NADPH. At the maximal rate observed, 60% of delta 1-pyrroline-5-carboxylate produced is recycled back to proline. This cycle provides a mechanism for transferring reducing equivalents from NADPH into mitochondria and is linked to glucose oxidation in the pentose pathway by NADPH turnover.
D000818:Animals; D002384:Catalysis; D002474:Cell-Free System; D003600:Cytosol / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D005954:Glucosephosphate Dehydrogenase / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D008930:Mitochondria, Liver / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D011392:Proline / Q000096:biosynthesis*; D011758:Pyrroles / Q000378:metabolism*; D011762:Pyrroline Carboxylate Reductases / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains
D011758:Pyrroles; C015485:delta-1-pyrroline-5-carboxylate; D009249:NADP; D011392:Proline; D005954:Glucosephosphate Dehydrogenase; D011762:Pyrroline Carboxylate Reductases; D005947:Glucose