Enhanced purine nucleotide synthesis in erythrocytes of uremic patients

Zusammenfassung

Erythrozyten von gesunden Kontrollpersonen und von urämischen Patienten wurden im Dichtegradienten in drei Altersklassen getrennt und die Enzyme des Purinnukleotid-Reutilisierungsstoffwechsels bestimmt:

• 1. Bei gleichzeitiger Verminderung der Fraktion der alten Zellen war der Anteil der jüngeren Erythrozyten in den urämischen Patienten um das drei- bis vierfache erhöht.

• 2. In allen Fraktionen der urämischen Erythrozyten waren die spezifischen Aktivitäten der vom Zellalter-abhängigen Enzyme, Phosphoribosyl-1-pyrophosphat (PRPP)-Synthetase, die Adenin-Phosphoribosyltransferase und die intrazellulären Phosphoribosyl-1-pyrophosphat-Konzentrationen deutlich erhöht. Die Halbwertszeiten und die kinetischen Parameter beider Enzyme waren jedoch unverändert.

• 3. Die Aktivität des Pentosephosphat-Shunts (PPS), gemessen an den intrazellulären Ribose-5-phosphat-Konzentrationen, war in den Erythrozyten urämischer Patienten nicht signifikant verändert. Nach Aktivierung des Pentosephosphat-Shunts durch Methylenblau (10⁻⁴ M) werden in allen Erythrozyten-Fraktionen gleiche Steigerungsraten festgestellt, wobei die intrazellulären Phosphoribosyl-1-pyrophosphat-Konzentrationen bei physiologischen Serumphosphatspiegeln unverändert bleiben.

• 4. Die Versuche zeigen, daß als Ursache erhöhter intraerythrozytärer ATP — Spiegel in Urämie-Patienten eine gesteigerte Adeninnukleotidsynthese durch einen verstärkten “salvage-pathway” angenommen werden kann.

• 5. Unsere Ergebnisse weisen darauf hin, daß die hypermetabolische Aktivität der Erythrozyten niereninsuffizienter Patienten Ausdruck einer jüngeren Erythrozyten-Population ist.

Summary

Studies were performed which suggest that the increased red cell levels of ATP and adenine nucleotides in uremic patients may be secondary to an increased biosynthetic rate of adenine nucleotides via salvage pathway by the predominance of younger red cells. Both normal and uremic red cells were separated in three groups according to their age by centrifugation in a discontinous density gradient using phthalate esters as separating liquids.

• 1. The uremic subjects have a substantial reduction of the older (more dense) red cells with a 3–4 fold increase of the younger red cells.

• 2. The specific activity of the salvage pathway enzymes phosphoribosyl-1-pyrophosphate (PRPP)-synthetase, adenine phosphoribosyltransferase and the red cell PRPP levels are strictly age-dependent and substantially increased in the uremic erythrocytes. No definite alterations of enzyme halflifes of both enzymes could be discerned. No differences were found in the substrate saturation curves between the enzymes of normal and uremic erythrocytes.

• 3. Red cell methylene-blue stimulated pentose phosphate pathway (PPS) wasn't significantly lower in patients with chronic uremia than in controls, as could be shown by the determination of red cell ribose-5-phosphate concentration. There was no difference of the steady-state levels of ribose-5-phosphate in the age-dependent red cell fractions of both normal and uremic subjects.

• 4. The results support the assumption that the elevated ATP levels in the uremic red cells may be related to their higher intrinsic ATP synthetic capacity via salvage pathway.

• 5. The enzymatic alterations in the uremic red cells are independent of extracorpuscular factors including phosphate, and it was concluded that most of the described abnormalities in the uremic red cell metabolism reflect only the hypermetabolic state of a young red cell population. The conflicting results concerning red cell metabolism of uremic patients could be probably caused by an inconstant percentage of younger red cells in various uremic patients.