The biosynthetic pathway of L-ascorbic acid in Euglena gracilis Z.
D-Glucose and D-galactose were the starting materials of L-ascorbic acid biosynthesis in Euglena gracilis as evidenced by feeding experiments of unlabeled and labeled sugars, but D-glucose was the more effective precursor. The addition of various acid derivatives of D-glucose and D-galactose, with the exception of D-glucono-delta-lactone, considerably augmented L-ascorbic acid formation. D-Galacturonic acid and L-galactono-gamma-lactone showed greater effects than did D-glucurono-gamma-lactone and L-gulono-gamma-lactone. The results of isotopic dilution experiments also showed the preference for the galacto-configuration. Fed U-14C-D-glucose was transformed into labeled D-galacturonic acid to a greater extent than into labeled D-glucuronic acid, and added D-galacturonic acid only caused extensive accumulation of labeled D-galacturonic acid. These results together show that the pathway involving D-galacturonic acid and L-galactono-gamma-lactone is the major one, the one involving D-glucuronic acid L-gulono-gamma-lactone being the minor one. A likely pathway for L-ascorbic acid biosynthesis in Euglena is proposed in the Scheme, which thus involves uronic acid intermediates and configurational inversion.
D000818:Animals; D001205:Ascorbic Acid / Q000096:biosynthesis*; D002237:Carbohydrate Dehydrogenases / Q000378:metabolism; D005056:Euglena gracilis / Q000378:metabolism*; D005690:Galactose / Q000031:analogs & derivatives / Q000378:metabolism; D005947:Glucose / Q000031:analogs & derivatives / Q000378:metabolism; D005965:Glucuronates; D007783:Lactones; D013379:Substrate Specificity; D013401:Sugar Alcohol Dehydrogenases / Q000378:metabolism; D014533:Uridine Diphosphate Glucose Dehydrogenase / Q000378:metabolism; D014574:Uronic Acids / Q000096:biosynthesis
D005965:Glucuronates; D007783:Lactones; D014574:Uronic Acids; D002237:Carbohydrate Dehydrogenases; D013401:Sugar Alcohol Dehydrogenases; D014533:Uridine Diphosphate Glucose Dehydrogenase; D005947:Glucose; D001205:Ascorbic Acid; D005690:Galactose