Molecular basis for the deficiency in humans of gulonolactone oxidase, a key enzyme for ascorbic acid biosynthesis.

The inability of humans to synthesize L-ascorbic acid is known to be due to a lack of L-gulono-gamma-lactone oxidase, an enzyme that is required for the biosynthesis of this vitamin. Isolation of a cDNA for rat L-gulono-gamma-lactone oxidase allowed us to study the basic defect underlying this deficiency at the gene level and led to isolation of a human genomic clone related to L-gulono-gamma-lactone oxidase as well as three overlapping clones covering the entire coding region of the rat L-gulono-gamma-lactone oxidase cDNA. Sequence analysis study indicated that the human L-gulono-gamma-lactone oxidase gene has accumulated a large number of mutations since it stopped being active and that it now exists as a pseudogene in the human genome.