Leishmania donovani encodes a functional enzyme involved in vitamin C biosynthesis: Arabino-1,4-lactone oxidase

Abstract

Plants and most animals can synthesize ascorbate (vitamin C) for their own requirements, but humans have lost this ability during evolution. The last step in the biosynthesis of l-ascorbic acid involves the conversion of an aldonolactone substrate to ascorbate (or analogues), reactions catalyzed by a family of flavoprotein aldonolactone oxidase/dehydrogenases. We report cloning, molecular characterization, localization and functional importance of arabinonolactone oxidase (LdALO), an enzyme from L. donovani, a protozoan parasite that causes visceral leishmaniasis. L. donovani arabinonolactone oxidase gene is 1509-bp and encodes a putative 502-amino acid protein with a molecular mass of 57-kDa. A 57-kDa protein was obtained by heterologous expression of LdALO in Escherichia coli. Recombinant arabinonolactone oxidase (LdALO) obeys Michaelis–Menten kinetics utilizing d-arabinono-γ-lactone as a substrate, a property characteristic of the yeast enzyme. Activity towards the mammalian substrate, l-gulono-γ-lactone, could not be detected. The inhibitor study profile suggested the essentiality of cysteine residues for the activity of this enzyme. LdALO displayed glycosomal localization as in other kinetoplastids. Overexpression of LdALO in L. donovani resulted in better ability of survival of the parasite within the host in comparison to the vector transfectants. d-Arabinono-γ-lactone oxidase required for synthesizing ascorbate in Leishmania could be considered as a therapeutically exploitable target.