Simultaneous occurrence of xanthine oxidase and sulfite oxidase deficiency. A molybdenum dependent inborn error of metabolism?

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In a 3 -week old female child with clinical features including neurologic abnormalities and lens dislocation , xanthinuria co -existed with increased excretion of sulfur compounds (sulfite , S-sulfocysteine , taurine and thio-sulfate ). Low xanthine oxidase and absent sulfite oxidase activities were found on liver biopsy . No abnormality was detected in either parent . Both the above enzymes are molybdenum-flavoproteins . Normal serum molybdenum concentration seemed to rule out dietary deficiency or impaired absorption . A defect in the incorporation of the metal into flavoproteins is postulated in this case .