When studying nucleotide biosynthesis, especially in cells deficient in the salvage pathway, tracking isotope incorporation from labeled amino acids helps reveal the sources of nitrogen atoms in purine rings. Feeding cells with ^15N-labeled amino acids allows researchers to determine which amino acids contribute nitrogen to purines during de novo synthesis.

This article explains which amino acid among aspartic acid, glycine, glutamine, and aspartamine is NOT likely to produce ^15N-labeled purines when supplied to such cells.

Purine Biosynthesis and Nitrogen Sources

Purine rings are assembled stepwise from simple precursors, incorporating nitrogen atoms from specific amino acids:

• Glycine contributes both carbon and nitrogen atoms to the purine ring.

• Glutamine donates nitrogen atoms via its amide group in multiple steps.

• Aspartic acid provides nitrogen through its amino group during the formation of the purine ring.

• These amino acids are essential nitrogen donors in the de novo purine biosynthesis pathway.

Role of the Salvage Pathway

• The salvage pathway recycles free purine bases and nucleosides to form nucleotides, conserving energy.

• Cells deficient in the salvage pathway rely entirely on de novo synthesis, making isotope incorporation studies more straightforward.

Analysis of the Amino Acids

• Aspartamine is an amine derivative of aspartate but is not a standard amino acid involved in purine biosynthesis.

• It is not incorporated into purine rings as a nitrogen source.

• Therefore, feeding cells with ^15N-labeled aspartamine will not result in labeled purines.

Conclusion

In cells deficient in the nucleotide salvage pathway, feeding with ^15N-labeled amino acids will label purines if the amino acid is a nitrogen donor in purine biosynthesis. Among the options:

• Aspartic acid, glycine, and glutamine are nitrogen donors and will produce ^15N-labeled purines.

• Aspartamine is not involved in purine nitrogen incorporation and will not label purines.

Correct answer: (4) Aspartamine