Hagedorn C H, Phang J M
Arch Biochem Biophys. 1983 Aug;225(1):95-101. doi: 10.1016/0003-9861(83)90010-3.
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.
利用无细胞实验系统提供了脯氨酸循环的直接证据，该系统能将[1-³H]葡萄糖中的³H依次转移至NADP⁺、Δ¹-吡咯啉-5-羧酸，并生成[³H]脯氨酸。[³H]脯氨酸的形成取决于NADP、Δ¹-吡咯啉-5-羧酸、葡萄糖-6-磷酸脱氢酶和Δ¹-吡咯啉-5-羧酸还原酶的存在。在线粒体存在的情况下，由未标记的脯氨酸生成[³H]脯氨酸，为脯氨酸循环的一整轮提供了直接证据，该循环将戊糖途径中葡萄糖氧化产生的还原当量转移至线粒体。在这个循环中，脯氨酸被线粒体脯氨酸氧化酶氧化为Δ¹-吡咯啉-5-羧酸。Δ¹-吡咯啉-5-羧酸从线粒体释放出来，并通过Δ¹-吡咯啉-5-羧酸还原酶伴随着NADPH的氧化再循环回脯氨酸。在所观察到的最大速率下，产生的Δ¹-吡咯啉-5-羧酸中有60%再循环回脯氨酸。这个循环提供了一种将还原当量从NADPH转移至线粒体的机制，并通过NADPH周转与戊糖途径中的葡萄糖氧化相联系。
