THE STEREOSPECIFICITY, PURIFICATION, AND CHARACTERIZATION OF AN NADH-FERRICYANIDE REDUCTASE FROM SPINACH LEAF PLASMA MEMBRANE

摘要

The stereospecificity of NADH-fenicyanide reductase activities in the inner mitochondrial membrane, peroxisomal membrane, plasma membrane and tonoplast are all specific for the beta-hydrogen of NADH whereas the reductases in the ER, the Golgi and the outer mitochondrial membrane are alpha-specific. This shows unequivocally that the NADH-fenicyanide activity in the plasma membrane is not caused by ER contamination. In all the membranes one or several polypeptides with an apparent size of 45-50 kDa cross-react with antibodies raised against a microsomal NADH-ferricyanide reductase. An NADH-ferricyanide reductase was purified from spinach leaf plasma membranes. The enzyme was released from the membrane by CHAPS solubilization and purified 360-fold by ion-exchange chromatography followed by affinity chromatography and size exclusion chromatography on FPLC. A major band of 45 kDa was detected by SDS-PAGE and it cross-reacted with the anti-NADH-ferricyanide reductase antibodies. The native size of the enzyme is 160 kDa as determined by size-exclusion chromatography indicating that it is a tetramer. Isoelectric focusing revealed three isoenzymes between pH 5.3 and 5.6. The enzyme shows typical FAD fluorescence spectra with excitation peaks at 371 and 468 nm and an emission peak at 525 nm. It is specific for the beta-hydrogen of NADH and prefers NADH over NADPH as electron donor. It is highly specific for ferricyanide as electron acceptor and it is therefore unlikely to be the enzyme responsible for iron reduction on the outer surface of the plasma membrane. [References: 50]