Inhibition of glutaminyl cyclase alters pyroglutamate formation in mammalian cells

Mammalian cell lines were examined concerning their Glutaminyl Cyclase (QC) activity using a HPLC method. The enzyme activity was suppressed by a QC specific inhibitor in all homogenates. Aim of the study was to prove whether inhibition of QC modifies the posttranslational maturation of N-glutamine and N-glutamate peptide substrates. Therefore, the impact of QC-inhibition on amino-terminal pyroglutamate (pGlu) formation of the modified amyloid peptides Aβ(N3E-42) and Aβ(N3Q-42) was investigated. These amyloid-β peptides were expressed as fusion proteins with either the pre–pro sequence of TRH, to be released by a prohormone convertase, or as engineered amyloid precursor protein for subsequent liberation of Aβ(N3Q-42) after β- and γ-secretase cleavage during posttranslational processing. Inhibition of QC leads in both expression systems to significantly reduced pGlu-formation of differently processed Aβ-peptides. This reveals the importance of QC-activity during cellular maturation of pGlu-containing peptides. Thus, QC-inhibition should impact bioactivity, stability or even toxicity of pyroglutamyl peptides preventing glutamine and glutamate cyclization.