Formation Pathways and Opioid Activity Data for 3‐Hydroxypyridinium Compounds Derived from Glucuronic Acid and Opioid Peptides by Maillard Processes

Abstract

The kinetics of formation and identity of the reaction products of the glucuronic acid with three representative opioid peptides were investigated in vitro. Peptides were conjugated with glucuronic acid either in solution or under dry-heating conditions. From the incubations performed in solution N-(1-deoxy-D-fructofuranos-1-yluronic acid)-peptide derivatives (Amadori compounds) were isolated, whereas from the dry-heated reactions products containing the 3-hydroxypyridinium moiety at the N-terminal of the peptide chain were obtained. Experiments performed under mild dry-heating conditions (40 degrees C) in model systems based on Leu-enkephalin and glucuronic acid, and in environment of either 40% or 75% relative humidity, revealed that the higher level of humidity promoted a process that enhanced 3-hydroxypyridinium compound generation. The mechanism of 3-hydroxypyridinium formation is discussed. In comparison with their respective parent peptides, the N-(1-deoxy-D-fructofuranosyl-uronic acid) derivatives of the opioid peptides showed three- to 11-fold lower mu- and delta-receptor-binding affinities and agonist potencies in the functional assays, likely as a consequence of the steric bulk introduced at the N-terminal amino group. The further decrease in opioid activity observed with the 3-hydroxypyridinium-containing peptides may be due to the lower pK(a) of the 3-hydroxypyridinium moiety and to delocalization of the positive charge in the pyridinium ring system.

Glycosylation of lysine-containing pentapeptides by glucuronic acid: new insights into the Maillard reaction

“…In support of this hypothesis, it was shown that enkephalin/GlcA-derived Amadori compounds decompose readily to 3-hydroxypyridinium compounds under similar dry-heating conditions. 18 Ring-substituted pyridinium compounds were also previously obtained by reaction of either triose sugars or glucose with lysine and arginine residues in phosphate buffer solu- tions. 20,21 An interesting case was the formation of a conjugated enol-keto-immonium derivative in the reaction of sodium glucuronate with the lysine-containing tripeptide, Ac-Tyr-Lys-Gly-NH 2 .…”

“…16,17 To gain more detailed insight into peptide/protein glycosylation processes under physiological conditions, model systems using mixtures of GlcA with peptides and proteins containing selected structural elements should be studied. In our previous study, 18 we characterized the glycosylation products generated from GlcA and endogenous opioid peptides (enkephalins) that did not contain lysine residues. Herein we focus on the small lysine-containing peptides, Lys-Gly-Gly-Phe-Leu (1), Gly-Lys-Gly-Phe-Leu (4) and Ac-Gly-Lys-Gly-Phe-Leu (6) as model peptides to examine their susceptibility to nonenzymatic glycation reaction and to characterize the products formed after being exposed to glucuronic acid.…”

Metal-Binding Ability of Leu-Enkephalin, Related Glycoconjugates and Peptidomimetics

“…Amadori (4) were synthesized under the conditions described by Horvat et al (1998) [32] and Horvat et al (2007). [33] …”

“…[34] (5) was obtained as described by Horvat et al (2007). [33] All compounds were desalted using an octadecylsilica solid-phase extraction (SPE) cartridge (500 mg, 2.8 mL).…”

“…[34] (5) was obtained as described by Horvat et al (2007). [33] All compounds were desalted using an octadecylsilica solid-phase extraction (SPE) cartridge (500 mg, 2.8 mL). The following compounds all purchased from Sigma-Aldrich were used as metal ions source: Al(ClO4)3 × 10H2O, Ca(ClO4)2 × 4H2O, Cu(ClO4)2 × 6H2O, Zn(ClO4)2 × 6H2O and Pb(ClO4)2 × 3H2O.…”