Characterization of phosphorus content of biological samples by ICP-DRC-MS: potential tool for cancer research

Abstract

Phosphorus and sulfur are detected as phosphorus oxide and sulfur oxide ions (PO⁺ and SO⁺), produced by oxidation reactions with O₂ performed in the reaction cell of the inductively coupled plasma dynamic reaction cell mass spectrometer (ICP-DRC-MS), at sub-ng mL⁻¹ detection limits. This allows pM mL⁻¹ detection of phospho-proteins, with S used as an internal standard (see ref. 9). The method was applied to digests (in HCl) of in-vitro tyrosine kinase assays, both as an evaluation of kinase autophosphorylation and phosphorylation of substrate. Detecting the phosphorus/sulfur ratio (via measured PO⁺/SO⁺) in cell cultures is shown to provide a distinguishable difference between malignant cell lines and primary cultures. The PO⁺/SO⁺ ratio for human colorectal adenocarcinoma (CRC) tissue samples compared with matched normal (N) tissue samples from the same patients is shown to be higher, at (PO⁺/SO⁺)_CRC/(PO⁺/SO⁺)_N = 1.75 ± 0.18 (n = 4). Samples used in this analysis were of needle biopsy amounts (0.2–0.5 mg), with a greater than 70% tumor burden in CRC. The phospho-protein phosvitin is detected directly from dried one-dimensional polyacrylamide gels using laser ablation ICP-MS by detecting phosphorus at sub-nM amounts. The phosphorus detection limit for direct ablation, assessed from ablating gel doped with P and blank gel, is 0.6 µg g⁻¹ in gel. Direct detection of sulfur from the gels is obscured by the high sulfur background for the blank gels, which is attributed to the sulfur-containing catalysts used in polymerization and to sodium dodecyl sulfate (SDS) used for protein denaturing.