The uptake of pyrroline 5-carboxylate. Group translocation mediating the transfer of reducing-oxidizing potential.
The cellular uptake of pyrroline 5-carboxylate (P5C) is of interest because this nutritionally responsive constituent of human plasma can mediate the transfer of oxidizing potential into cells and stimulate the production of phosphoribosyl pyrophosphate. Using a cloned line of Chinese hamster ovary cells, we found that the uptake of P5C was saturable, temperature-dependent, and sensitive to metabolic inhibitors. Furthermore, this uptake of P5C exhibited unusual features. It was independent of sodium ion and had a pH optimum of 6.4. The kinetics characteristics of P5C uptake included an apparent Km of 0.46 +/- 0.04 mM and a Vmax of 19.6 +/- 1.8 nmol/min/mg. Although the Vmax for P5C was comparable to those for certain other amino acids, e.g. leucine, it was significantly higher than that for alpha-methylaminoisobutyric acid in these cells. Importantly, there was no interaction between these amino acids and the uptake mechanism for P5C. Twenty naturally occurring amino acids, each at a concentration of 5 mM, were without effect on the uptake of P5C. Interestingly, the uptake mechanism for P5C is unusual in that it is linked to the transfer of reducing-oxidizing potential. Over wide ranges of P5C concentration and duration of incubation, P5C entry is coupled to its conversion to proline and the concomitant oxidation of reduced pyridine nucleotide with stimulation of the pentose phosphate shunt. In fact, no free P5C derived from the medium could be detected in cells. Our interpretation of these findings is that P5C uptake occurs by its own unique mechanism, a group translocation that mediates the transfer of reducing-oxidizing potential.