Glucose-6-phosphate dehydrogenase from the blood cells of two antarctic teleosts: correlation with cold adaptation.

Glucose-6-phosphate dehydrogenase (G6PD) has been purified from the blood of two Antarctic teleost species, i.e., from the erythrocytes of Dissostichus mawsoni (family Nototheniidae), and from the plasma and cells of haemoglobinless Chionodraco hamatus (family Channichthyidae). The specific activities in haemolysates of Antarctic blood cells appear higher than that of a lysate of human erythrocytes. The two Antarctic enzymes have an apparent subunit molecular mass slightly higher than that of human G6PD; the electrophoretic behaviour on cellulose acetate is similar. Both Antarctic enzymes are irreversibly heat inactivated through a biphasic process. Km for glucose-6-phosphate (G6P) does not vary significantly with temperature, whereas Km for NADP increases at increasing temperature, kcat increases with temperature, with a break point at 35 degrees C (in human G6PD, the break point is at 15 degrees C). Thermodynamic and kinetic characterisation indicate that the catalytic performance of the enzyme of cold-adapted fish, at temperatures typical of their habitat, is more efficient than that displayed by G6PD from a temperature organism.