Kinetics and mechanism of the reaction of aminoguanidine with the alpha-oxoaldehydes glyoxal, methylglyoxal, and 3-deoxyglucosone under physiologicalconditions

Aminoguanidine (AG), a prototype agent for the preventive therapy of diabet ic complications, reacts with the physiological alpha-oxoaldehydes glyoxal, methylglyoxal, and 3-deoxyglucosone (3-DG) to form 3-amino-1,2,4-triazine derivatives (T) and prevent glycation by these agents in vitro and in vivo. The reaction kinetics of these alpha-oxoaldehydes with AG under physiologi cal conditions pH 7.4 and 37 degrees was investigated. The rate of reaction of AG with glyoxal was first order with respect to both reactants; the rat e constant k(AG,G), was 0.892 +/- 0.037 M-1 sec(-1). The kinetics of the re action of AG with 3-DG were more complex: the rate equation was d[T](o)/dt (initial rate of T formation) = [3-DG](k(AG,3-DG)[AG] + k(3-DG)), where k(A G.3-DG) = (3.23 +/- 0.25) x 10(-3) M-1 sec(-1) and k3-DG = (1.73 +/- 0.08) x 10(-5) sec(-1). The kinetics of the reaction of AG with methylglyoxal wer e consistent with the reaction of both unhydrated (MG) and monohydrate (MG- H2O) forms. The rate equation was d[T](o)/dt = {k(1)k(AG,MG)/(k(-1) + k(AG, MG)[AG]) + k(AG.MG-H2O)}[MG-H2O][AG], where the rate constant for the react ion of AG with MG, k(AG,MG), was 178 +/- 15 M-1 sec(-1) and for the reactio n of AG with MG-H2O, k(AG,MG-H2O), was 0.102 +/- 0.001 M-1 sec(-1); k(1) an d k(-1) are the forward and reverse rate constants for methylglyoxal dehydr ation MG-H2O reversible arrow MG. The kinetics of these reactions were not influenced by ionic strength, but the reaction of AG with glyoxal and with methylglyoxal under MG-H2O dehydration rate-limited conditions increased wi th increasing phosphate buffer concentration. Kinetic modelling indicated t hat the rapid reaction of AG with the MG perturbed the MG/MG-H2O equilibriu m, and the ratio of the isomeric triazine products varied with initial reac tant concentration. AG is kinetically competent to scavenge the alpha-oxoal dehydes studied and decrease related advanced glycated endproduct (AGE) for mation in vivo. This effect is Limited, however, by the rapid renal elimina tion of AG. Decreased AGE formation is implicated in the prevention of micr ovascular complications of diabetes by AG. BIOCHEM PHARMACOL 60;1:55-65, 20 00. (C) 2000 Elsevier Science Inc.