Occurrence, General Constitution and Properties of Molybdenum Enzymes

Abstract

Molybdenum is known to be an essential constituent of six enzymes: aldehyde oxidase, nitrate reductase, nitrogenase, sulphite oxidase, xanthine dehydrogenase and xanthine oxidase. Of these, four have a fad† component and all six contain iron, either as a cytochrome or an iron-sulphur moiety. The enzymes have high molecular weights, in the range 10⁵–10⁶. In most of them there are two atoms of molybdenum per molecule of protein so that the molybdenum content is only about 10⁻² per cent. This is one of the factors that has made the study of molybdenum-containing enzymes difficult. The occurrence of two molybdenum atoms per molecule of protein may be a reflection of the strong tendency of low molecular weight molybdenum compounds to exist as dimers, although it could be coincidental since there is no evidence for molybdenum-molybdenum interactions in any of the enzymes. The mammalian enzymes: aldehyde oxidase, sulphite oxidase, xanthine dehydrogenase and xanthine oxidase have been isolated from liver, but milk is the best source of xanthine oxidase. Bacteria are the chief sources of nitrate reductase and nitrogenase. The properties of a representative group of molybdenum-containing enzymes are summarised in table 21.