Measurement of blood protease kinetic parameters with self-assembled monolayer ligand binding assays and label-free MALDI-TOF MS

We report novel ligand binding assay (LBA) surface modalities that permit plasma protease catalytic efficiency (k_cat/k_m) determination by MALDI-TOF MS without the use of liquid chromatography or internal standards such as chemical or metalized labels. Two model LBAs were constructed on planar self-assembled monolayers (SAMs) and used to evaluate the clinically relevant metalloprotease ADAMTS-13 kinetics in plasma. The SAM chemistries were designed to improve biosampling efficiency by minimization of nonspecific adsorption of abundant proteins present at ∼100 000× the concentration of the endogenous enzyme. In the first protocol, in-solution digestion of the ADAMTS-13 substrate (vWFh) was performed with immunoaffinity enrichment of the reaction substrate and product to SAM arrays. The second configuration examined protease k_cat/k_m via a surface digestion modality where different substrates were covalently immobilized to the SAM at controlled surface density for optimized protease screens. The results show the MALDI-TOF MS LBA platforms provide limits of quantitation to ∼1% protease activity (∼60 pM enzyme concentration) in <1 h analysis time, a ∼16× improvement over other MS-based LBA formats. Implementation of a vacuum-sublimed MALDI matrix provided good MALDI-TOF MS intra- and interday repeatability, ∼1.2 and ∼6.6% RSD, respectively. Platform reliability permitted k_cat/k_m determination without internal standards with observed values ∼10× improved versus conventional fluorophoric assays. Application of the assays to 12 clinical plasma samples demonstrated proof-of-concept for clinical applications. Overall, this work demonstrates that rationally designed surface chemistries for MALDI-TOF MS may serve as an alternative, label-free methodology with potential for a wide range of biotechnology applications related to targeted enzyme molecular diagnostics.