Streptozotocin (STZ) is a 2-deoxy-D-glucopyranose derivative of a class of drugs known as alkylnitrosoureas, and is an established diabetogenic agent whose cytotoxic affects on pancreatic β-cells has been partially explained by the presence of its N-methyl-N-nitrosourea side chain, which has the ability to release nitric oxide as well as donate methyl groups to nucleotides in DNA. It has also been observed that STZ administration results in a rise in the level of O-GlcNAcylated proteins within β-cells. Not coincidentally, STZ has also been shown to directly inhibit the O-GlcNAcase activity of the enzyme NCOAT in vitro, which is the only enzyme that possesses the ability to remove O-GlcNAc modifications on proteins in the nucleus and cytosol. Since O-GlcNAc modification plays a role on a number of proteins in a vast amount of cellular processes, this shift in whole-cell protein O-GlcNAcylation state affords another source of cell death. We set about to find the exact mechanism by which STZ inhibits O-GlcNAcase activity. Inhibition is achievable because the GlcNAc analog STZ targets the active site of the enzyme whereby it is catalyzed. During this process, the enzyme converts STZ to a compound that closely resembles the natural ligand transition state, but is distinctly more stable energetically. As a result, this analog is catalyzed to completion at a much slower rate, thereby out-competing GlcNAc substrate for the active site, and inhibiting the enzyme. © 2005 Elsevier Inc. All rights reserved.