The present invention provides novel physiological substrates of mammalian glutaminyl cyclase (QC, EC 2.3.2.5), new effectors of QC, methods for screening for such effectors, and the use of such effectors and pharmaceutical compositions comprising such effectors for the treatment of conditions that can be treated by modulation of QC-activity. Preferred compositions additionally comprise inhibitors of DP IV or DP IV-like enzymes for the treatment or alleviation of conditions that can be treated by modulation of QC- and DP IV-activity.