Cross-linking of proteins by aldotriose: reaction of the carbonyl function of the keto amines generated in situ with amino groups

Publisher Website
Google Scholar
Abstract
Nonreductive modification of proteins with glyceraldehyde forming 2-oxo-3-hydroxypropylated protein is mechanistically analogous to nonenzymic glycation reactions. The latent cross-linking potential of glyceraldehyde as a consequence of the reactivity of the carbonyl function of 2-oxo-3-hydroxypropyl groups of nonreductively modified protein has been now investigated. Reaction of RNase A (0.5 mM) with glyceraldehyde (20 mM) at pH 7.4 and 37.degree. C for 4 h resulted in the intermolecular cross-linking of the protein, with the concomitant development of a yellow chromophore with two new absorption bands having maxima around 305 and 375 nm. The product exhibited a fluorescence that had excitation and emission maxima around 365 and 450 nm, respectively. The presence of NaCNBH3 during the reaction, which selectively reduces the Schiff base adducts of aldotriose to form 2,3-dihydroxypropyl groups on proteins, inhibited both the cross-linking reaction and the development of the absorption and fluorescence characteristics. The hydroxymethyl group of the aldotriose is not an essential moiety since the cross-linking potential of glyceraldehyde is comparable to that of glyceraldehyde 3-phosphate. The formation of cross-links appears to involve the carbonyl function of the keto amines resulting in the formation of Schiff base adducts (ketimine linkages) as the initial event. Consistent with this, incubation of 2-oxo-3-hydroxypropylated RNase A with [14C]glycine ethyl ester resulted in the incorporation of the reagent into the protein. The cross-linking reaction was inhibited when the reaction of RNase A with glyceraldehyde was carried out in the presence of amino compounds, such as glycine ethyl ester, ethanolamine, glucosamine, and aminoguanidine. An equimolar amount of aminoguanidine inhibited the nonreductive incorporation of [14C]glyceraldehyde into RNase A by nearly 85%. The inhibition of the cross-linking reaction by the aminoguanidine and other amino compounds is predominantly a consequence of the inhibition of the nonenzymic glycation of RNase A. The results of the present study demonstrate that the protein cross-linking by aldotriose under physiological condition is latent and is a consequence of the reactivity of the carbonyl function of keto amines generated in situ with the amino groups of protein.