Dehydrogenase involved in the conversion of monodictyphenone to the prenyl xanthones such as emericellin, shamixanthone and epishamixanthone (PubMed:21351751, PubMed:22730213). Monodictyphenone is first converted to variecoxanthone A via a paeciloxanthone intermediate by the consecutive actions of the FAD-dependent monooxygenase mdpD and the xanthone prenyltransferase xptB (PubMed:21351751). XptB catalyzes regular O-prenylation at the hydroxy group of C-7 of the xanthone ring (PubMed:23150454). Variecoxanthone A is further prenylated to emericellin by xptA before being reduced to shamixanthone and epishamixanthone by the dehydrogenase xptC (PubMed:21351751). Secondary metabolite biosynthesis. Impairs the production of the xanthones shamixanthone, emericellin and epishamixanthone; and accumulates variecoxanthone A (PubMed:21351751). Belongs to the tryptophan dimethylallyltransferase family.