Gulonolactone oxidase is missing in teleost fish. The direct spectrophotometric assay

Data in the literature imply that some fish species evolved with the capacity to synthesize ascorbic acid. Gulonolactone oxidase activity has been reported in kidney and/or liver tissues. However, it is shown here that this microsomal enzyme activity is missing in common carp hepatopancreas and kidney, whereas high activity was confirmed in pigeon kidney, rat liver, bovine liver and amphibian (Xenopus) kidney tissues. A new assay using either the whole tissue homogenate or microsomes solubilized by sodium deoxycholate was developed to directly measure the formation of ascorbic acid spectrophotometrically. Identical values were found using this assay as well as the assay in which formed ascorbate was determined by the dinitrophenyl hydrazine (DNPH) method. In some experiments, these results were confirmed by polarographically measured oxygen consumption.