β-Lipoprotein- and LDL-associated serum γ-glutamyltransferase in patients with coronary atherosclerosis

Elevation of serum gamma-glutamyltransferase (GGT) activity is a risk factor for myocardial infarction and stroke. GGT activity can catalyze the oxidation of low-density lipoprotein (LDL), a process involved in the pathogenesis of atherosclerosis. Serum GGT is partially adsorbed onto circulating LDL, and catalytically active GGT has been found within atherosclerotic plaques, colocalizing with oxidized LDL and foam cells. We investigated the the nature of the LDL-associated GGT, the degree of correlation between total serum GGT levels and β-lipoprotein (β-LP)-associated GGT, and whether this association is altered in subjects with coronary artery disease (CAD). LDL-bound GGT showed an entire, amphiphilic heavy chain, but the association was easily lost during LDL purification by affinity chromatography. When the activity of GGT associated with polycation-precipitated β-lipoproteins was assayed, an identical immunoreactive GGT was found in Western blot, and a statistically significant linear correlation was found between total serum GGT levels and the corresponding β-LP-bound activities (p < 0.0001) in controls and patients with CAD. Nevertheless, subjects with CAD presented a lower ratio of β-LP-bound GGT to total serum GGT respect to controls (p < 0.05) and healthy subjects with elevated serum GGT (p < 0.01). In addition, a relative decrease of total serum GGT was observed in CAD subjects of older age as compared to younger ones (p < 0.005).