A copper chelating agent suppresses carbonyl stress in diabetic rat lenses

Abstract

To clarify whether transition metals are involved in carbonyl stress in diabetic tissues, we observed the effects of a metal chelating agent, trientine (TE) hydrochloride on the levels of methylglyoxal (MG), 3-deoxyglucosone (3-DG), advanced glycation end products, 8-hydroxy-2′-deoxyguanosine (8-OHdG), and polyol pathway metabolites along with semicarbazide-sensitive amine oxidase (SSAO) enzyme activity in lenses from streptozotocin-induced diabetic rats. Lens MG and 3-DG levels were significantly higher in diabetic rats than nondiabetic controls, and TE significantly restored the increase of these compounds. Lens argpyrimidine was also increased in diabetic rats as compared with controls and was significantly reduced by TE. Lens SSAO activity and 8-OHdG were also significantly elevated in diabetic rats, and TE suppressed both of them, whereas TE showed no effect on the polyol pathway metabolites. The results indicate that transition metals play a significant role in the formation of MG and 3-DG via oxidative stress and SSAO activity.

Introduction

Carbonyl compounds such as methylglyoxal (MG) and 3-deoxyglucosone (3-DG) are recognized as most potent precursors of advanced glycation end products (AGEs), and the so-called carbonyl stress has recently attracted attention as the etiology of diabetic micro- and macroangiopathy (Baynes & Thorpe, 1999, McCance et al., 1993). Advanced glycation end products and dicarbonyl compounds are also known to contribute to the formation of cataract by aging (Ahmed et al., 1997, Shamsi et al., 1998) or by diabetes (Lyons et al., 1991, Sady et al., 2000). Of further interest, dicarbonyl compounds have been shown to cause direct damage to DNA as well (Murata-Kamiya & Kamiya, 2001, Shimoi et al., 2001), indicating a more potential role of the compounds in the development of diabetic complications.

Multiple synthetic routes of carbonyl compounds, from glucose and other carbohydrates or from lipids, require transition metals such as copper or iron. Transition metals are known to mediate the autoxidation of reducing sugars, the glycoxidation of Amadori products (Thornalley, Langborg, & Minhas, 1999), and the oxidation of lipids as well. These oxidative processes are involved in the formation of carbonyl compounds such as glyoxal (Fu et al., 1996). In addition, copper ion is essential for the activities of semicarbazide-sensitive amine oxidase (SSAO) that exerts as a synthetic enzyme for MG (Callingham, Crosbie, & Rous, 1995).

In accordance with these facts, previous studies showed that d-penicillamine, a chelating agent for copper, reduced AGEs in bovine lenses (Stevens, 1995) or human erythrocytes (Jakus, Hrnciarova, Carsky, Krahulec, & Rietbrock, 1999) incubated in high glucose medium. The results seem to indicate a tangible role of transition metals in the formation process of AGEs, yet it remains to be clarified if the reduction of AGEs is attributable to the suppression of carbonyl compounds by the chelating agent.

In present study, we observed effects of trientine (triethylenetetramine, TE), a copper chelating agent clinically used for treatment of Wilson's disease (Dahlman et al., 1995), on the levels of MG and 3-DG along with AGEs in lenses from streptozotocin-induced diabetic and nondiabetic rats. Lenses were chosen because the involvement of transition metals in lens AGE formation had been reported (Stevens, 1995). The activity of lens SSAO and the level of 8-hydroxy-2′-deoxyguanosine (8-OHdG) level, an indicator of oxidative stress (Fraga, Shigenaga, Park, Degan, & Ames, 1990), were also examined to elucidate the mechanisms by which TE may affect the carbonyl stress. The effects of TE on lens sorbitol and fructose were investigated as well, considering the possible involvement of the polyol pathway in the formation of 3-DG (Lal et al., 1995).