Pyridine nucleotides interconversion

Interconversion of cortisone and cortisol catalyzed by llfSHSDHl is fully reversible in vitro. The physiologic direction in vivo is cortisone reduction, which is driven by a high [NADPH]/[NADP+] ratio in the ER lumen. The high ratio can be maintained only by local NADP+ reduction catalyzed by certain luminal dehydrogenases, such as the H6PDH (15, 16, 24), because the permeability of the ER membrane to pyridine nucleotides is negligible (17, 18). 

Ribose phosphates phosphorylated derivatives of ribose. Ribose is phosphorylated in position 5 by the action of ribokinase (EC 2.7.1.15) and ATP ribose 5-phosphate is also produced in the Pentose phosphate cycle (see), and in the Calvin c cle (see) of photosynthesis. Phosphoribomutase cat yses the interconversion of ribose 5-phospbate and ribose 1-phosphate, and the cosubstrate of this reaction is ribose l,5-f>isphosphate. 5-Phosphoribosyl 1-pyrophos-phate donates a ribose 5-phosphate moiety in the de novo biosynthesis of purine and pyrimidine nucleotides (see Purine biosynthesis. Pyrimidine biosynthesis), in the Salvage pathway (see) of purine and pyrimidine utilization, in the biosynthesis of L-Histi-dine (see) and L-Tryptophan (see) and in the conversion of nicotinic acid into nicotinic acid ribotide (see Pyridine nucleotide cycle). Ribose 1-phosphate can also take part in nucleotide synthesis (see Salvage pathway). 

Another mechanism for the interconversion of the pyridine nucleotides involves the enzyme pyridine nucleotide transhydrogenase, which has been purified from extracts of Pseudomonas jiuorescens. - It catalyzes a number of reactions, of which two are given below. (The reduced coenzymes are represented as TPNH and DPNH.)... 

But in mammalian species, several mechanisms exist for transferring NADH reducing equivalents, i.e., the malate-aspartate cycle (10) and the a-glycerophosphate-dihydroxy acetone phosphate cycle (16). It seems unlikely that the magnitude of NADH redox transfers due to the interconversions of proline - P5C would alter the N AD+/NADH balance. The oxidation of NADPH by P5C reductase, on the other hand, may play a major role in regulating NADP+/NADPH. Importantly, the of P5C reductase for NADPH is markedly lower than that for NADH (86,117). Although the V .. activities are higher with NADH than with NADPH the conversion of P5C to proline by PC reductase would affect NADP / NADPH more than NAD+/NADH if one considers the respective in vivo concentrations and the respective redox ratios of the two pyridine nucleotides. 

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