Methods to analyze transglutamination of proteins involved in apoptosis

Enhanced expression of transglutaminases is a frequent, though not obligatory phenomenon in apoptosis, which is associated with cells dying in steady interaction with their tissue environment. Modification of cellular proteins by transglutamination is a tightly controlled procedure. If transglutamination is dysregulated, it has profound and potentially detrimental consequences on cellular functioning. Under conditions normally occurring in living cells transglutaminase activity is usually undetectably low (latent) and can only be tested by careful preselection of proteins of interest. In late stages of apoptosis, transglutaminases can become rampant in dying cells and a minuscule fraction of dead cells may overshadow many more living ones, which may cause inherent and severe methodological and interpretation bias. Therefore, in this chapter, we describe the analysis of dead cell remnants for protein-bound transglutaminase-mediated cross-link content. In the techniques described below, we rely on the increasing availability and user-friendliness of mass spectrometric equipments.