The oxidative mechanism of heparin interferes with radical production by glucose and reduces the degree of glycooxidative modifications on human serum albumin.

Abstract

Among substances which may prove useful in preventing or reducing the progression of glycooxidative modifications of proteins, heparin plays a unique role. To elucidate the mechanism whereby heparin may favourably influence the protein structure during glycation, human serum albumin (HSA) was glycated with both 25 and 50 mM glucose in the absence and presence of 12 microg.mL(-1) low-molecular-mass heparin. Glycation caused: (a) modifications of fluorescence emission and excitation spectra consistent with the covalent attachment of glucose to protein; (b) a significant increase in the esterase activity of HSA on p-nitrophenyl acetate; (c) a reduced susceptibility to tryptic digestion and (d) enhanced formation of high-molecular mass aggregates of HSA. These alterations were accompanied by oxidative reactions, as the EPR spectra showed a clear-cut radical signal, dependent on glucose concentration, further confirmed by measurement of the carbonyl content of HSA, as an indirect proof of oxidative damage. In the presence of heparin all the above alterations, especially at 25 mM glucose, turned out to be antagonized. The effects of heparin were dependent on its specific binding to HSA, which triggered an oxidative mechanism strikingly different from that caused by glucose. In the presence of heparin, only the radical species catalyzed by heparin was detected across all samples of glycated HSA, irrespective of glucose concentration. In addition, at 25 mM glucose, enhancement of the oxidative capacity of heparin was also observed. The results demonstrate that the oxidative mechanism sustained by heparin mediates biological effects that may be beneficial in reducing the extent of glycooxidative damage on HSA.