molybdenum cofactor sulfurase
The MOCOS gene provides instructions for making an enzyme called molybdenum cofactor sulfurase. This enzyme is necessary for the function of two other enzymes, xanthine dehydrogenase and aldehyde oxidase. Xanthine dehydrogenase is involved in the normal breakdown of purines, which are building blocks of DNA and its chemical cousin, RNA. Specifically, it carries out the final two steps in the process: the conversion of a molecule called hypoxanthine to another molecule called xanthine, and the conversion of xanthine to uric acid, a waste product that is normally excreted in urine and feces. Less is known about the function of aldehyde oxidase, although it appears to play a role in the breakdown (metabolism) of many different compounds.
Molybdenum cofactor sulfurase carries out a chemical reaction that adds sulfur to a molecule called the molybdenum cofactor. This molecule is required for xanthine dehydrogenase and aldehyde oxidase to be turned on (activated) and carry out their functions.
At least four mutations in the MOCOS gene have been found to cause hereditary xanthinuria type II, a condition that most often affects the kidneys. Most of these mutations change a single protein building block (amino acid) in molybdenum cofactor sulfurase. The effects of these mutations are not fully understood, but they likely alter the shape and function of the enzyme. If molybdenum cofactor sulfurase is unable to add sulfur to the molybdenum cofactor, xanthine dehydrogenase and aldehyde oxidase are not activated. The loss of aldehyde oxidase activity does not appear to cause any signs or symptoms. However, the loss of xanthine dehydrogenase activity prevents the conversion of xanthine to uric acid, leading to an accumulation of xanthine in the kidneys and other tissues. The excess xanthine can form tiny crystals that accumulate in the kidneys, occasionally leading to the formation of stones that can impair kidney function and ultimately cause kidney failure. Less commonly, xanthine crystals build up in the muscles, causing pain and cramping. In some people with hereditary xanthinuria type II, the condition does not cause any health problems.