Showing papers by "Jean L. Johnson published in 1998"

Human sulfite oxidase R160Q: identification of the mutation in a sulfite oxidase-deficient patient and expression and characterization of the mutant enzyme.

Abstract: Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480- and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center Kinetic analysis of the R160Q protein showed an increase in Km for sulfite combined with a decrease in kcat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant kcat/Km Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum

Ahomocysteinemia in molybdenum cofactor deficiency

Abstract: We report an infant with molybdenum cofactor deficiency (MCD) and a unique clinical presentation of hemiplegia, hypotonia, dystonia, and bilateral basal ganglia changes. Biochemistry revealed absent serum homocysteine, low concentrations of plasma cystine, high levels of urinary S-sulfocysteine and sulfite, and high levels of oxypurines in serum and urine. The depletion of cysteine and cystine through reaction with sulfite suggests that other thiols and thiol-dependent proteins may be similarly depleted. Ahomocysteinemia may be a clue to the mechanism of cytotoxicity in MCD.