Galactose Oxidase

Galactose oxidase (GAO) produced by the fungus Dactylium dendroides (Nobles and Madhosingh 1963) has found its main use in the quantitative determination of galactose in blood and other biological fluids (Frings and Pardue 1964; Ford and Haworth 1964; Hankin 1966; Roth et al. 1965). Hjelm and Tengstrom (1968) reported a comparison of methods using the oxidase and the dehydrogenase for blood galactose concluding that the two were equally good.

Because GAO oxidizes galactose even in polysaccharides it has been used to locate galactose histochemically (Roberts and Gupta 1965) and to detect and distinguish glycoproteins (Itaya et al. 1975). The enzyme has been reviewed by Malmström et al. (1975).

Characteristics of Galactose Oxidase from Dactylium dendroides:

Molecular weight: 68,000 ± 3,000 (Kosman et al. 1974).

Composition: GAO appears to be the only copper protein which contains a single "nonblue" Cu²⁺ atom per molecule. It has been characterized in detail by Kosman et al. (1974). See also Kelly-Falcoz et al. (1965) and Cleveland et al. (1975).

Optimum pH: 7.0 (Cooper et al. 1959).

Extinction coefficient: = 15.4 (Cooper et al. 1959).

Inhibitors: Cyanide, diethyldithiocarbamate, azide and hydroxylamine (Cooper et al. 1959). See also Kwiatkowski and Kosman (1973).

Specificity: GAO will oxidize galactose and some galactose derivatives in both free and polymeric form. Oxidation occurs at the C₆ position. Cleveland et al. (1975); Amaral et al. (1963) and Avigad et al. (1961) present data on the comparative reactivities of galactose, its derivatives and polymers. Zancan and Amaral (1970) showed that dihydroxyacetone is an equally good substrate. Hamilton et al. (1973) report that to a small degree both glycerol and salicyl alcohol are substrates.

Stabilizers: Sucrose provides stability. It may be removed by dialyzing against 0.001 M Cu²⁺.

Stability: Native GAO is very stable, though not the apoenzyme. Dialysis against cupric sulfate is used in purification. (Kosman et al. 1974).