Absence of which of the following enzyme in humans is responsible for the inability to synthesize ascorbic acid?: (A) L-Glucuronic acid oxidase, (B) L-Gulonic acid reductase, (C) L-Gulonolactone oxidase, (D) L-Gulonolactone reductase
Answer is C. Uronic acid pathway is an alternate pathway for oxidation of glucose without ATP formation. Fig: Uronic acid pathway An intermediate (L-gluconate) of this pathway can be used for the synthesis of ascorbic acid in plants by enzyme L-gulonolactone oxidase. However, In human beings and other primates, as well as guinea pigs, bats, and some birds and fishes, ascorbic acid cannot be synthesized because of the absence of L-Gulonolactone oxidase.