Urinary proteins from stone formers promote calcium oxalate crystallization, growth and aggregation via oxidative...

Various urinary parameters are used for determining kidney stone risk. However, almost all of the widely used lithogenic indices rely on urinary concentrations of small molecules/ions and pH.

To address whether urinary macromolecules (especially oxidatively modified proteins) also play a critical role in determining the stone risk.

Complexed urinary proteins (proteome) were purified from healthy individuals and calcium oxalate (CaOx) stone formers and performed various crystal assays and quantitative proteomics to compare them. Bioinformatic analyses were performed to gain additional insights, and the obtained data were verified by ELISA.

While the normal urinary proteome inhibited CaOx stone-forming mechanisms (i.e., crystallization, growth and aggregation), the stone formers' urinary proteome promoted all these CaOx crystal parameters. Descriptive proteomics by nanoLC-ESI-LTQ-Orbitrap-MS/MS analysis identified 203 and 381 proteins in the urine of healthy individuals and stone formers, respectively. Analyses of physicochemical properties revealed only molecular mass and isoelectric point that slightly increased in the stone formers' urine, whereas instability index, grand average of hydrophathicity (GRAVY) and amino acid composition were comparable. Interestingly, proportion of oxidatively modified proteins (particularly those with methionine oxidation, methionine dioxidation and cysteine trioxidation) markedly increased (∼2.5-fold) in the stone formers' urine. Quantitative proteomics revealed 89 increased and 56 decreased proteins in the stone formers' urine. The oxidized proteins had a greater proportion (>3-fold) in the increased proteins (77 %) compared with the decreased ones (23 %), whereas the non-oxidized proteins showed comparable proportions (54 % and 46 %, respectively). Functional enrichment analyses revealed a correlation between the increased proteins and oxidative stress biological processes and molecular functions. Finally, ELISA confirmed the significantly increased levels of oxidized proteins in the stone formers' urine compared with that of healthy individuals.

These data implicate that oxidatively modified proteome serves as a key pathogenic factor or risk for CaOx kidney stone formation.