Occurrence in yeast of L-galactonolactone oxidase which is similar to a key enzyme for ascorbic acid biosynthesis in animals, L-gulonolactone oxidase.

l-Galactonolactone oxidase is believed to catalyze the last step of l-ascorbic acid biosynthesis in yeast. A highly purified preparation of this enzyme from baker's yeast was obtained by a seven-step procedure. The molecular weight of the purified enzyme was estimated to be 290,000 by gel filtration, while the dissociated enzyme possessed a molecular weight of 56,000, based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme catalyzes the reaction, l-galactono-γ-lactone + O₂ → l-ascorbic acid + H₂O₂. l-Gulono- and d-altrono-γ-lactone also serve as substrates. The enzyme was found to contain a flavin which is covalently bound to the enzyme protein. By comparing the properties of this enzyme with those of isofunctional enzymes of higher plants and animals, it became evident that the yeast enzyme is more like the l-gulonolactone oxidase (EC 1.1.3.8) of animals than the l-galactonolactone dehydrogenase (EC 1.3.2.3) of higher plants. Since phylogenetically lower animals are reported to lack l-gulonolactone oxidase, the finding of a similar enzyme in yeast is of great interest.