Sulfite oxidase catalyses the physiologically vital oxidation of sulfite to sulfate, the terminal reaction in the degradation of the sulfur containing amino acids, cysteine and methionine.

The physiological importance of an active sulfite oxidase was emphasized by the discovery of a child apparently lacking hepatic sulfite oxidase. Genetic deficiency related to human sulfite oxidase is associated with severe clinical abnormalities namely mental retardation, seizures, characteristic dysmorphic features and dislocated lenses. The urine of patient contains abnormally large amount of S-sulfocysteine, sulfite and thiosulfate and virtually no inorganic sulfate, making the enzyme of biomedical importance.

The main objectives of the work presented in this thesis were, screening of some plant leaves and different tissues of goat for the presence of sulfite oxidase, isolation and purification of the enzyme from the source showing highest enzyme activity and physico-biochemical characterization of the enzyme.

Liver was found to have highest activity amongst all goat tissues in which the sulfite oxidase activity was determined. Similarly, leaves of Syzygium cumini showed highest activity amongst all five plant leaves screened for sulfite oxidase activity. Sulfite oxidase from goat liver has been purified to homogeneity by using number of techniques. Ninety- three fold purification was achieved with a yield of 7%.

The absorption spectrum of enzyme indicated the presence of heme in the sulfite oxidase. Circular dichroic spectra showed the characteristic spectra of proteins in far UV region. Secondary structures of sulfite oxidase were also calculated to be 45 % a-helix, 9% b-sheet anb 26% b-turn.

The size of sulfite oxidase was found to be 113 kDa and size of subunits of the enzyme was found to be 56 kDa. Sulfite oxidase is composed of two subunits of equal size. The hydrodynamic properties, stokes' radius was determined to be 5.01 nm and frictional ratio, ¦/¦o, was found to be 1.57.

The pH and temperature optima for sulfite oxidase were found to be 8.5 and 25 °C respectively. Sulfite oxidase showed appreciably less activity in phosphate and carbonate buffers when compared with Tris-Cl buffer under same conditions.

Kinetic parameters, Km and Vmax were found to be 6.98 ´ 10-4 M and 0.5824 nmoles/mL/min respectively. Ferricyanide was 8 times more effective electron acceptor than, physiological electron acceptor, cytochrome c. A number of salts had inhibitory effect on sulfite oxidase activity and inhibition caused by these salts were mixed non-competitive type.

Atomic absorption spectroscopy indicated the presence of molybdenum and sodium dithionite reduced sulfite oxidase showed characteristic spectrum of b5 heme, indicating the presence of heme in goat liver sulfite oxidase.

Limited sequence analysis from N-terminal end was done. The sequence was found to be trp-glu-pro-ser-gly-ala.

Sulfite oxidase from plant leaves of Syzygium cumini was partially purified. The absorption spectrum indicated that heme prosthetic group is not present in plant sulfite oxidase. The Km and Vmax for plant sulfite oxidase were found to be 1.38 ´ 10-3 M and 19.23 nmoles/mL/min respectively. Analysis revealed that there is no homology between animal and plant sulfite oxidases.