Malondialdehyde, a lipid-derived aldehyde alters the reactivity of Cys34 and the esterase activity of serum albumin.

Malondialdehyde (MDA), one of the key end products of lipid oxidation is elevated in a variety of diseases. It is now well established that MDA can modify proteins in vivo. This paper describes the effects of modification of albumin by MDA and peroxidized linolenic acid on the reactivity of Cys34, a crucial residue conferring antioxidant properties. BSA (10 mg/ml) was incubated with MDA (1 mM) for 72 h in phosphate buffer (100 mM, pH 7.4). BSA was also incubated for three days with lipid samples, which have already undergone peroxidation for 2, 5, 7, and 9 days respectively. The reactivity of Cys34 after modification was monitored using cystamine and 5,5'-dithiobis(2-nitro benzoic acid) (DTNB). The Kobs for the reaction was found to be different between native and MDA modified protein clearly indicating that modification affects the reactivity of Cys34. The individual rate constant (K1) for reaction with DTNB varied significantly between albumin and modified albumin suggesting that loss in reactivity was due to changes at Cys34. However, (K2), the rate constant for reaction of protein with cystamine, determined from a plot of Kobs versus cystamine concentration did not change. This study further shows that modification results in significant loss of the esterase like activity of albumin. Since albumin plays a crucial role in the antioxidant defence due to its abundance (approximately 0.6 mM) in serum, these findings have implications in disease states where increased levels of MDA and oxidative stress drastically may affect the antioxidant capacity of serum.