Variability of protein content in calcium oxalate monohydrate stones

Background and Purpose: Urinary stones are heterogeneous in their fragility to lithotripter shockwaves. As a first step in gaining a better understanding of the role of matrix in stone fragility, we measured extractible protein in calcium oxalate monohydrate (COM) stones that were extensively characterized by micro-computed tomography (micro CT). Materials and Methods: Stones were scanned using micro CT (Scanco mCT20, 34 μm). They were ground, and the protein extracted using four methods: 0.25M EDTA, 2% SDS reducing buffer, 9M urea buffer, and 10% acetic acid. Protein was measured using NanoOrange. The SDS extracts were also examined using polyacrylamide electrophoresis (PAGE). Results: Extracted protein was highest with the SDS or urea methods (0.28% ± 0.13% and 0.24% ± 0.11%, respectively) and lower using the EDTA method (0.17% ± 0.05%; P < 0.02). Acetic acid extracted little protein (0.006 ± 0.002%; P < 0.001). Individual stones were significantly different in extractability of protein by the different methods, and SDS-PAGE revealed different protein patterns for individual stones. Extracted protein did not correlate with X-ray-lucent void percentage, which ranged from 0.06% to 2.8% of stone volume, or with apatite content. Conclusions: Extractible stone-matrix protein differs for individual COM stones, and yield is dependent on the extraction method. The presence of X-ray-lucent voids or minor amounts of apatite in stones did not corelate with protein content. The amounts of protein recovered were much lower than reported by Boy ce, show-ing that these methods extracted only a fraction of the protein bound up in the stones. The results suggest that none of the methods tested will be useful for helping to answer the question of whether matrix content differs among stones of differing fragility to lithotripter shockwaves.