PHYSIOLOGIA PLANTARUM 115: 213–220. 2002 Copyright C Physiologia Plantarum 2002 Printed in Denmark – all rights reserved ISSN 0031-9317 Non-enzymatic protein modification by the Maillard reaction reduces the activities of scavenging enzymes in Vigna radiata U. M. Narayana Murthy*, Yongheng Liang, Prakash P. Kumar and Wendell Q. Sun Department of Biological Sciences, National University of Singapore, Kent Ridge Crescent, Singapore 119260 *Corresponding author, e-mail: scip8012/nus.edu.sg Received 30 July 2001; revised 27 November 2001 The non-enzymatic modification of proteins through the Mail- lard reaction plays an important role in the loss of seed viabil- ity during seed storage. In the present study we examined whether the Maillard reaction reduces the activities of scav- enging enzymes in Vigna radiata (mung bean) seeds during storage. Seeds were stored under various conditions for differ- ent duration. Maillard products were monitored by measuring protein fluorescence, and the activities of glutathione re- ductase (GR), superoxide dismutase (SOD), ascorbate per- oxidase (APX), catalase (CAT) and peroxidase (POX) were Introduction Seeds progressively lose their viability during prolonged storage, and loss of seed viability is highly dependent on temperature and seed moisture content (Roberts 1972, Priestley 1986). Membrane deterioration is one of the major events in the loss of seed viability during storage (Priestley 1986, Bewley and Black 1994). The occurrence of damage to cellular membranes during the ageing pro- cess is manifested by an increase of solute and electrolyte leakage from seeds during imbibition (Parrish and Leo- pold 1978, Ray and Gupta 1979, Tully et al. 1981). It is generally accepted that the loss of viability with seed ageing is mainly connected with the loss of plasma mem- brane integrity (Senaratna et al. 1988), although a series of other deteriorative changes such as damage to pro- teins and nucleic acids occurs (reviewed by McDonald 1999). Modification of membrane composition and function affects seed quality by decreasing seed germi- nation and normal development (Wilson and McDonald 1986, Halistones and Smith 1988, Pukacka and Kuppier 1988, Clark and Synder 1991). Seed deterioration is thought to be associated with an accumulation of active forms of oxygen such as superox- ide radical, hydrogen peroxide (H 2 O 2 ) and hydroxyl rad- ical, which may initiate reactions on polyunsaturated Physiol. Plant. 115, 2002 213 determined. The accumulation of Maillard products in seed axes increased during storage with increasing moisture con- tent and temperature, and was correlated with the decline in seed vigour. The activities of GR, CAT and APX decreased in proportion to the increase in Maillard products at all the moisture contents and temperatures tested. These enzymatic changes were also correlated with seed vigour. However, the activities of SOD and POX remained unchanged and ap- peared to be less sensitive to the Maillard reaction. fatty acids leading to lipid peroxidation and various kinds of cellular damage (Priestley 1986, Wilson and McDonald 1986). Antioxidant defense systems in plants include free radical and peroxide scavenging enzymes such as superoxide dismutase (SOD), catalase (CAT) and enzymes of the ascorbate-glutathione cycle (Bowler et al. 1992, Foyer et al. 1994). This oxide reduction cycle involves the enzymes ascorbate peroxidase (APX) and glutathione reductase (GR). Therefore, the mechanisms of free radical and peroxide generating and scavenging might be correlated to the longevity of seeds during storage. Among other changes involved in the loss of seed viability, non-enzymatic protein modification by the Maillard reaction also plays an important role. These modifications can occur by non-enzymatic glycation with reducing sugars or by reaction with aldehydes pro- duced from free radical-mediated lipid oxidation (Priestley and Leopold 1983, Priestley 1986, Wettlaufer and Leopold 1991, Sun and Leopold 1995, Murthy and Sun 2000). The Maillard reaction may contribute to seed ageing through the chemical alteration of proteins, thus depressing metabolic capability, and reducing the ability of the metabolic system to limit free radical damage and