Antibody-free workflows for protein quantification by LC-MS/MS.

Antibody-free approaches for quantitative LC-MS/MS-based protein bioanalysis are reviewed and critically evaluated, and compared with the more widely used immunoaffinity-based approaches. Antibody-free workflows will be divided into four groups and discussed in the following order: direct analysis of signature peptides after proteolytic digestion; enrichment of target proteins and signature peptides by fractionated protein precipitation; enrichment of target proteins and signature peptides by reversed-phase and ion-exchange solid-phase extraction; and enrichment of target proteins and signature peptides by (antibody-free) affinity-solid-phase extraction.