Endogenous reductase activities for the generation of ribitol-phosphate, a CDP-ribitol precursor, in mammals.

The core M3 O-mannosyl glycan on α-dystroglycan serves as the binding epitope for extracellular matrix molecules. Defects in core M3 glycans cause congenital muscular dystrophies that are collectively known as dystroglycanopathies. The core M3 glycan contains a tandem D-ribitol-5-phosphate (Rbo5P) structure, which is synthesized by the Rbo5P-transferases fukutin (FKTN) and fukutin-related protein (FKRP) using CDP-ribitol (CDP-Rbo) as a donor substrate. CDP-Rbo is synthesized from CTP and Rbo5P by CDP-Rbo pyrophosphorylase A (CRPPA). However, the Rbo5P biosynthesis pathway has yet to be elucidated in mammals. Here, we investigated the reductase activities toward four substrates including ribose, ribulose, ribose-phosphate, and ribulose-phosphate to identify the intracellular Rbo5P production pathway and elucidated the role of the aldo-keto reductases AKR1A1, AKR1B1, and AKR1C1 in those pathways. It was shown that the ribose reduction pathway is the endogenous pathway that contributes most to Rbo5P production in HEK293T cells and that AKR1B1 is the major reductase in this pathway.