Purification and characterization of sulfite oxidase from goat liver

Sulfite oxidase (EC 1.8.3.1) catalyzes the physiologically vital oxidation of sulfite to sulfate, the terminal reaction in the degradation of sulfur containing amino acids. Genetic deficiency related to human sulfite oxidase is associated with the severe clinical abnormalities with no effective therapies known, making the enzyme of immense biomedical importance. In the present study, sulfite oxidase was been purified from the goat tissues, a hitherto unexplored source, in particular from the liver, and its physico and biochemical properties were characterized. The liver was chosen as it showed the highest activity, compared to kidney and muscle. The enzyme was purified to homogeneity by salting out, gel filtration and ion-exchange chromatography. It was a dimer (113 kDa) having two identical subunits (56 kDa) and did not contain free sulfhydryl groups. Its spectral analysis showed the presence of heme and molybdenum. circular dichroism (CD) spectra in near and far-UV regions showed the presence of significant amounts of secondary structures (45% alpha helix, 9% beta structure and 26% beta turn and remaining random coil) in the native molecule. The kinetic and hydrodynamic properties of the enzyme were also determined. Results also showed that ferricyanide was 8-times more effective electron acceptor than its physiological acceptor cytochrome c. The limited N-terminal analysis of the enzyme revealed the sequence up to six amino acids Trp-Glu-Pro-Ser-Gly-Ala. Together, these results suggested the liver was a major source of sulfite oxidase in goat and most of its physico-chemical, except secondary structure and amino acid sequence from N-terminal and biological properties were fairly similar to the sulfite oxidase isolated from other mammalian species/organs.