Alpha-thiolamines such as cysteine and cysteamine act as effective transglycating agents due to formation of irreversible thiazolidine derivatives

Abstract

Non-enzymatic glycation of proteins and some phospholipids is considered to be an important factor in the genesis of diabetic complications. While this process has been viewed traditionally as entirely non-enzymatic and unidirectional, the discovery of fructosamine-3-phosphate (FN3K) and identification of FN3K-mediated deglycation mechanisms have made it apparent that non-enzymatic glycation is not unidirectional and that it can be reversed by deglycation reactions. While FN3K operates on ketosamines, the second intermediate in the non-enzymatic glycation cascade, we recently identified another potential deglycation mechanism that can operate on Schiff bases, the first intermediates of the non-enzymatic glycation process. The initial step in this postulated deglycation process is a transglycation reaction between a L.M.W. intracellular nucleophiles and a macromolecule-bound aldosamines, which regenerate unmodified proteins or phospholipids with a concomitant production of aldose-nucleophile transglycation byproducts. In vitro, transglycation occurs readily with amino acids, polyamines, thiols and thiolamines. There are indications that this reaction also occurs in vivo since in an initial GC/MS analysis of human urine we detected significant amounts of a transglycation product, glucose-cysteine (G-Cys), which was markedly increased in diabetics. Despite these encouraging early data, it is not yet clear to what extent transglycation is important in vivo and which intracellular nucleophiles are most relevant to this process. As discussed by us previously in this journal, one likely candidate for this role is glutathione since it is distributed universally and since there are well described mechanisms for removal of S-linked glutathione adducts from cells by the multi-drug-resistance (MDR) pumps. In this paper we report on another class of likely transglycating agents, alpha-thiolamines such as cysteine and cysteamine. While concentrations of these compounds in tissues are significantly lower than those of GSH, they react with Schiff bases more rapidly than GSH and, most significantly they form stable and irreversible thiazolidine products such as glucose-cysteine (G-Cys) and glucose-cysteamine (G-Ctm) that can subsequently be removed from cells. The possibility that alpha-thiolamines may play a physiological role as deglycating agents in vivo is very attractive since it suggests a possible strategy for inhibiting nonenzymatic glycation and diabetic complications that could be readily implemented through nutritional or pharmacological approaches. Such intervention is eminently feasible since there are at least three thiolamines already approved for human use. These include cysteamine used for the treatment of cystinosis; N-acetylcysteine utilized as a mucolytic and antioxidant agent, in the therapy of acetaminophen poisoning and radiocontrast-induced nephrotoxicity; and penicillamine used for treatment of Wilson's disease. Consequently, determining whether these compounds have the expected anti-glycating effects in vivo should be relatively straightforward.