Glutaminyl cyclases (QC; EC 188.8.131.52) are enzymes present in animals and plants, catalyzing the conversion of N-terminal glutaminyl residues into pyroglutamic acid. Mammalian QCs are involved in maturation of several hormones. The QC-forms of plants and animals display no homology in terms of primary structure. The characteristics of the plant QC from Carica papaya and of human QC were compared in the dissertation. New continuous enzymatic assays were developed and human QC was expressed heterologously in the methylotrophic yeast Pichia pastoris. Plant QC was purified from papaya latex. Human and papaya QC show a similar catalytic specificity towards oligopeptides and a similar pH-dependence of catalysis. Both QC-forms catalyze the cyclization of N-terminal glutamyl residues under mildly acidic pH. Differences, however, were observed in conversion of dipeptides and dipeptide analogs. Human QC was inhibited by imidazole derivatives and heterocyclic chelators. QC-apoenzyme was reactivated by transition metal ions, e.g. zinc ions, revealing human QC as a metal-dependent enzyme. Papaya QC was neither inhibited by imidazoles, nor by chelators. Hence, plant QCs catalyze metal independent. The results point to a similar catalytic mechanism of QCs from plants and animals in general. The metal dependent catalysis of human QC could indicate an interaction of the γ-carbonyl group of the N-terminal glutaminyl residue of the substrate with the active site-bound metal ion, leading to acceleration of the intramolecular cyclization reaction. The conversion of N-terminal glutamyl residues by human QC could be responsible for generation of amyloidogenic pGlu3-Aβ peptides, which play an important role in the initiation of Alzheimer's disease. Thus, human QC is suggested as a target for drug development.