ORIGINAL ARTICLE A comparative study of the chemical reactivity of pyridoxamine, Ac-Phe-Lys and Ac-Cys with various glycating carbonyl compounds Miquel Adrover Bartolome ´ Vilanova Juan Frau Francisco Mun ˜oz Josefa Donoso Received: 27 February 2008 / Accepted: 26 April 2008 Ó Springer-Verlag 2008 Abstract Pyridoxamine (PM) has long been known to inhibit protein glycation via various mechanisms of action. One such mechanism involves the scavenging of carbonyl compounds with glycating ability. Despite the abundant literature on this topic, few quantitative kinetic studies on the processes involved have been reported. In this work, we conducted a comparative kinetic study under physiological pH and temperature conditions of the reactions of PM, Ac-Phe-Lys and Ac-Cys with various glycating carbonyl compounds (viz. aldehydes, a-oxoaldehydes and ketones). The microscopic formation rate constants for Schiff bases of PM and various carbonyl compounds, k 1 , are of the same order of magnitude as those for the Schiff bases of Ac-Phe- Lys. However, because PM exhibits a higher proportion of reactive form at physiological pH, its observed second- order rate constant is ca. five times greater than that for Ac-Phe-Lys. That could explain PM ability to compete with amino residues in protein glycation. On the other hand, the observed formation rate constant for thiohemi- acetals is four orders of magnitude greater than the formation constants for the Schiff bases of PM, which excludes PM as a competitive inhibitor of Cys residues in protein glycation. Keywords Pyridoxamine (PM)  Carbonyl compounds  Kinetics  Shiff base  Protein Glycation Introduction The non-enzymatic protein glycation and the formation of its end-products, so-called Advanced Glycation End Products (AGEs), are extremely important in hyperglyce- mic people; as such, they have been the subject of much research over the past 30 years (Ulrich and Cerami 2001; Thorpe and Baynes 2003; Horvat and Jakas 2004; Aldini et al. 2007). Protein glycation has been deemed responsible for a wide range of diabetes-associated pathologies including some eye diseases (Stitt 2005), renal dysfunc- tions (Bohlender et al. 2005), arteriosclerosis (Kume et al. 1995) and Alzheimer’s disease (Reddy et al. 2002). As shown in Fig. 1 for D-glucose, protein glycation begins with the condensation of a terminal amino group in a side chain of a Lys or Arg residue with a carbohydrate or some other carbonyl compound. This initially gives a Schiff base which undergoes reversible rearrangement to Amadori compounds that ultimately produces AGEs via a complex body of reactions (Ulrich and Cerami 2001). Other reactive carbonyl species can be produced from oxidative degradation of sugars, Schiff bases, and Amadori compounds (Thornalley et al. 1999) and can also contribute to protein glycation. Propagation of protein damage may also be mediated by reactive oxygen species which can be formed from oxi- dative degradation of sugars (Wolff and Dean 1987), Schiff bases and Amadori compound (Mullarkey et al. 1990). These radical species can react with protein backbone and amino acid side chains (Pro, Thr, Trp, His and Cys) (Stadtman and Levine 2003), and contribute to the devel- opment of pathology in diabetes (Wolff 1993). Although the high D-glucose levels present in hyper- glycemic people is the main culprit of protein glycation, the presence of other carbohydrates such as the aldehydes M. Adrover  B. Vilanova (&)  J. Frau  F. Mun ˜oz  J. Donoso Institut Universitari d’Investigacio ´ en Cie `ncies de la Salut (IUNICS), Departament de Quı ´mica, Universitat de les Illes Balears, Cra. Valldemossa km 7.5, 07122 Palma de Mallorca, Spain e-mail: bartomeu.vilanova@uib.es 123 Amino Acids DOI 10.1007/s00726-008-0098-6