Novel glutaminyl-peptide cyclotransferase-like proteins (QPCTLs), which are isoenzymes of glutaminyl cyclase (QC, EC 2.3.2.5), and to isolated nucleic acids coding for these isoenzymes, all of which are useful for the discovery of new therapeutic agents, for measuring cyclase activity, and for determining the inhibitory activity of compounds against these glutaminyl cyclase isoenzymes.