Molybdenum cofactor (MoCo) is needed by three enzymes in humans: sulfite oxidase, xanthine oxidase and aldehyde oxidase. The pathway of its synthesis is so conserved that plants and bacteria can readily use human enzymes. Bacteria, however, diverge after the first three steps from this path and their final MoCo differs from that of the eukaryotes. Plants and animals have also developed a refinement of their MoCo which is needed for the function of their xanthine and aldehyde oxidases. This means, in humans we find sulfurated instead of desulfurated molybdenum cofactor on these two enzymes (Schwarz 2005; Schwarz, Mendel, Ribbe 2009).