released during allergic reactions. Monoamine oxidase (MAO) is important in serotonin degradation to excretable metabolites and in the metabolism of catecholamines (epinephrine, norepinephrine, and dopamine). MAO inhibitors are used as antidepressant drugs. Lysyl oxidase uses lysine and hydroxylysine found in collagen and elastin as substrates for posttranslational processing to produce cross-linkages needed for the development of connective tissues, including those of bone, lung, and the circulatory system.

Ferroxidases are copper enzymes found in plasma, with a function in ferrous iron oxidation (Fe²⁺→Fe³⁺) that is needed to achieve iron’s binding to transferrin (Linder and Hazegh-Azam, 1996). Ferroxidase I, also called ceruloplasmin, is the predominant copper protein in plasma and may also have antioxidant functions. Defects in ceruloplasmin function produce cellular iron accumulation, a result that supports its ferroxidase role (Harris and Gitlin, 1996). Ferroxidase II is found in human plasma, but it may have a role in iron metabolism in specific cellular sites. A transmembrane copper-containing protein (hephaestatin) with ferroxidase activity has been described (Pena et al., 1999; Vulpe et al., 1999). Cytochrome c oxidase is a multisubunit enzyme in mitochondria that catalyzes reduction of O₂ to H₂O. This establishes a high energy proton gradient required for adenosine triphosphate (ATP) synthesis. This copper enzyme is particularly abundant in tissues of greatest metabolic activity including heart, brain, and liver. Dopamine β monooxygenase uses ascorbate, copper, and O₂ to convert dopamine to norepinephrine, a neurotransmitter, produced in neuronal and adrenal gland cells. Dopa, a precursor of dopamine, and metabolites used in melanin formation are oxidatively produced from tyrosine by the copper enzyme tyrosinase. α-Amidating monooxygenase (α-AE), also called peptidylglycine α-AE, uses copper and ascorbate to remove two carbons from a C-terminal glycine of peptides, thus generating an amide. A number of peptide hormones are posttranslationally modified by α-AE (Harris, 1997).

Two forms of superoxide dismutase are expressed in mammalian cells, a mangano and cupro/zinc form (Harris, 1997). Copper/zinc superoxide dismutase (Cu/Zn SOD) uses two copper atoms for conversion of the superoxide anion (O₂^-+) to H₂O₂ and O₂. Zinc atoms have a structural role in the enzyme. The enzyme is localized in the cytosol and, along with the mitochondrial manganese-containing form, provides a defense against oxidative damage from superoxide radicals that, if uncontrolled, can lead to other damaging reactive oxygen species. Mutations in the Cu/Zn SOD gene, which alter the