Glucose (Hexose 6-Phosphate) Dehydrogenase in Liver of Rainbow Trout

Abstract

Abstract Liver of the rainbow trout and other salmonid fish contains a highly active glucose dehydrogenase which is also active toward glucose-6-P, galactose-6-P, 2-deoxyglucose-6-P, xylose, and galactose. It resembles one of the rat liver hexose-6-P dehydrogenase isozymes in its broad substrate specificity and its electrophoretic migration on starch gel; but differs in some kinetic properties. The enzyme, purified approximately 200-fold, is more active toward glucose than toward glucose-6-P at pH 7.4, but is less active toward glucose than glucose-6-P at pH 10. Km for glucose is 20 to 80 mm, depending on the pH; and for glucose-6-P it ranges from 0.01 to 0.1 mm. Over the pH range of 6.5 to 10.5 the glucose dehydrogenase activity is about the same for NAD+ or NADP+ as coenzyme, but Mg2+ ions are required with NADP+. The enzyme is much more active with NADP+ than NAD+ with glucose 6-phosphate as substrate, but activity with NADP+ is inhibited by Mg2+, whereas activity with NAD+ is unaffected. The enzyme appears to be equally divided between the particulate and soluble fractions. Trout liver also contains two other electrophoretically distinguishable glucose-6-P dehydrogenases, but which differ from this enzyme in their inactivity toward NAD+ and glucose.