INHIBITORY EFFECT OF NONENZYMATIC GLYCATION ON UBIQUITINATION AND UBIQUITIN-MEDIATED DEGRADATION OF LYSOZYME

We examined ubiquitination and ubiquitin-mediated degradation of glycated protein by rabbit reticulocyte lysate fraction II (ubiquitin-free preparation). Non-glycated lysozyme and three glycated lysozyme preparations with different glucose binding ratios were used as substrates. Glycation sites of the lysozyme were mostly the ε-NH2 group of lysine residues, since modification at the α-NH2 group of the amino terminal was not detectable. Ubiquitin was conjugated with three glycated lysozyme preparations, which contained 1.4, 2.8 and 4.5 mol glucose per mol, by fraction II supplemented with hemin. The extent of formed conjugates was reduced 81, 72 and 56% of those of ubiquitin-non-glycated lysozyme conjugates, respectively. Additionally, ubiquitin-mediated degradation of the resultant conjugates was reduced and their respective rates were 97, 56 and 19% of that of the non- glycated lysozyme. These results indicated that both ubiquitin conjugation and ubiquitin-mediated degradation of the lysozyme were inhibited by nonenzymatic glucose binding to the lysozyme. © 1993 Academic Press, Inc.