Properties of Purified Lens Transglutaminase and Regulation of its Transamidase/Crosslinking Activity by GTP

Abstract

On account of its protein crosslinking activity, the Ca2+-dependent transglutaminase of the lens is likely to be involved in the formation of cataracts. We have now purified the rabbit lens enzyme to near homogeneity as judged by SDS-PAGE (Mr∼78 kDa), and a key feature of the procedure was the use of a highly selective affinity chromatographic step with a fibronectin fragment as ligand. The catalytic activity of the lens transglutaminase, measured by the incorporation of dansylcadaverine into dimethylcasein, was compared with those of two similar enzymes isolated from human red cells and from guinea pig liver, respectively. All three enzymes were inhibited by GTP, but the lens enzyme was most sensitive to inhibition by the nucleotide. Moreover, GTP was also shown to inhibit the formation of the ∼55 kDa βcrystallin dimers in the Ca2+-treated rabbit lens homogenate, proving that the nucleotide is a negative regulator for the crosslinking activity of transglutaminase in this tissue.