1981 Volume 4 Issue 11 Pages 886-892
Gliclazide, an oral hypoglycemic drug having sulfonylurea structure, is stable in native human serum, but the drug is easily degraded in an albumin-removed serum. It was found that the degradation was inhibited by readdition of serum albumin. Then, the protein fractions possessing gliclazide-degrading activity were separated from the human serum on a gel filtration using Sephadex G-150 column, and some characterizations and kinetic analysis of the degradation were performed. Moreover, the protective effects of native and modified serum albumin (BSA) on the degradation were studied. The degrading factor was found in both protein fractions of IgM (fraction M) and IgG (fraction G), but was not found in a fraction of albumin (fraction A). The degradation of gliclazide by the fraction G was inhibited in proportion to the amount of albumin in the fraction A. Several commercial albumins, bovine serum albumin (fraction V) and human serum albumin (fraction V and fatty acid-free), showed also the protective effect on the degradation. The degrading activities of the fraction M and G were completely lost by a heat-treatment at 100°C for 10 min. The maximal rate of degradation was obtained by incubation at 37-40°C for 16-24 h. The dose-response curve for the degradation of gliclazide by the fraction G was represented as an enzyme-reaction like sigmoidal curve. The double-reciprocal plot for the degradation was a parabolic profile, suggesting to be n order reaction (n > 1) and Hill plot was a straight line with a slope of 3.0. Each kinetic parameter of reaction order (Hill constant, n), Michaelis constant (Km) and equilibrium constant (K) was 3.0, 17.8 mM and 1.74×10-4mM-n, respectively. Since the degradation rate highly correlated with the binding capacity of the secondray binding site on the BSA molecule (r=0.915), it was suggested that the protective effect of BSA on the degradation of gliclazide closely associated with the interaction of gliclazide in the secondary binding site on the BSA molecule.