In view of the recent identification of two instances of human sulfite oxidase deficiency, it has become necessary to study the expression of the enzyme in human fibroblasts. Since activity can be detected in low amounts, the possibility of inducing higher levels by supplementation of the medium with molybdenum and sulfur amino acids will be investigated. A sensitive radioisotopic assay for enzyme activity and a radioimmunoassay for the protein will be developed to help in the characterization of the molecular lesion in the human deficiency cases. The molecular mechanisms underlying the chemistry of the molybdenum cofactor of sulfite oxidase represent an area wherein a genetic lesion would result in a phenotype of sulfite oxidase deficiency. Since the process of sulfite oxidase activation by the cofactor involves a transfer of molybdenum from a carrier to sulfite oxidase protein, a technique involving the use of Mo99 will be developed for assaying the cofactor in fibroblasts. The possibility that the known instances of human sulfite oxidase deficiency could be the result of cofactor deficiency will be examined. Metallothionein and copper-chelatin are low molecular weight proteins which are induced in animals exposed to specific metals. Radioimmunoassays will be developed for detection of low levels of these proteins for investigating the possible biological functions of these proteins in normal animals and for detection of these proteins in cultured cells exposed to toxic metals. BIBLIOGRAPHIC REFERENCES: Waud, W.R., and Rajagopalan, K.V. Purification and properties of the NAD ion-dependent (Type D) and O2-dependent (Type O) forms of rat liver xanthine dehydrogenase, Arch. Biochem. Biophys. 172, 354 (1976). Waud, W.R., and Rajagopalan, K.V. The mechanisms of conversion of rat liver xanthine dehydrogenase from an NAD ion-dependent form (Type D) to an O2-dependent form (Type O), Arch. Biochem. Biophys. 172, 365 (1975).