valence:
the oxidation state of an element. For cationic metals, the
valence is the same as the number of positive charges it
carries. For species composed of a core element to which one
or more oxygen or halogen atoms is attached, the bonds are
highly polar. In such cases the valence is the number of
electrons covalently shifted towards the oxidizing element.
W
X
xanthine:
2,6-dioxopurine; an oxidation product of guanine or
hypoxanthine; a heterocyclic compound composed of two rings
of carbon and nitrogen atoms.
xanthine oxidase:
an oxidoreductase utilizing molybdenum (Mo), flavin (FAD),
and iron (FE) which converts xanthine to uric acid. Xanthine
is first hydrated and then dehydrogenated. Electrons pass
firstly to molybdenum, then to flavin, then to iron, and
finally to diatomic oxygen to produce superoxide.
xenobiotic:
a material foreign to the body such as a nonfood chemical
or drug; a synthetic chemical which is not naturally occuring
in the biosphere; a substance which induces or is altered
by the mixed function oxidase and/or conjugase system.
Xenobiotics often induce production of various reductases.
Many xenobiotics are eliminated by conjugation with glutathione.
Over time excessive exposure therefore depletes glutathione.
Some xenobiotics are metabolized into compounds which are
more toxic than the parent molecule.
Y
Z
zinc cation (Zn++):
a transition metal which in its divalent form serves to
activate numerous enzymes making Zn++ an important nutrient.
Examples include: alcohol dehydrogenase, carbonic anhydrase,
and superoxide dismutase. Zinc peroxide (ZnO2) has been used
topically to disinfect wounds.
