Photooxidative Activity of Stress-Induced Plant Proteins of WSCP Family

The paper examines the photochemical activity of the stress-induced WSCP proteins belonging to subclasses IIa (BoWSCP, from Brassica oleracea) and IIb (LvWSCP, from Lepidium virginicum). The proteins were found to oxidize the biologically important reductants, such as NADH and sodium ascorbate. Photooxidation of NADH and ascorbate was observed when air-saturated solutions of the in vitro-assembled holoforms of BoWSCP and LvWSCP were irradiated with red light (λ ≥ 650 nm). Photooxidation rate of the substrates was similar for both proteins. No photooxidation of NADH and ascorbate was observed in the absence of oxygen. Absorption spectroscopy and circular dichroism data revealed that photooxidation of these substrates did not result in any considerable destruction of chlorophyll molecules as well as its dimer structure in WSCP. A higher rate of ascorbate photooxidation as compared to NADH correlated with a higher rate constant of singlet oxygen quenching by ascorbate. The rate of photooxidation decreased in the presence of NaN3, an efficient singlet oxygen quencher. Our findings demonstrate that WSCP proteins are capable of photosensitized oxidation of exogenous substrates such as NADH and AscH. This process may possibly involve singlet oxygen generated by the triplet excited chlorophyll dimers within WSCP holoforms. The obtained results contribute to better understanding the protective function of WSCP proteins in plants under stress.