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A 7-month-old baby girl, the second child born to unrelated parents was brought to Pediatrics outdoor department. History revealed that she did not respond well to breast-feeding and was changed entirely to a formula based on cow’s milk at 4 weeks. Between 7 and 12 weeks of age, she was admitted to the hospital twice with a history of screaming after feeding, but was discharged after observation without a specific diagnosis. Elimination of cow’s milk from her diet did not relieve her symptoms; her mother reported that the screaming bouts were worse after the child drank juice and that she frequently had gas and a distended abdomen. The child was diagnosed having ‘Hereditary fructose intolerance’. The mother of the child was instructed to eliminate fructose containing foods from the child’s diet and was strictly instructed to feed milk without table sugar. The table sugar (sucrose), a disaccharide, contains glucose and fructose linked as:

A. O-α-D-glucopyranosyl-(1->6)-α -D- fructofuranoside

B. O-β-D-glucopyranosyl-(1->6)-α -D- fructofuranoside

C. O-α-D-glucopyranosyl-(1->2)-β -D-fructofuranoside

D. O-α-D-glucopyranosyl-(1->2)-α -D-fructofuranoside

E. None of the above.

The correct answer is C- O-α D-glucopyranosyl-(1->2)-β -D-fructofuranoside. The onset of symptoms after ingestion of juice (fructose or fructose containing diet) is a sign of hereditary fructose Intolerance’.

Hereditary fructose intolerance is caused by deficiency of Aldolase B, the enzyme required for the metabolism of fructose. These patients are healthy and asymptomatic until fructose or sucrose (table sugar) is ingested (usually from fruit, sweetened cereal, or sucrose-containing formula). Elimination of dietary fructose is both a compulsory and therapeutic step.

In patients who are ill, elimination may also serve as a diagnostic test because all symptoms should completely resolve. With this treatment, as the patient matures, symptoms become milder, even after fructose ingestion, and the long-term prognosis is good.

Table sugar (sucrose) is a source of fructose and in Sucrose, the anomeric carbon atoms of a glucose unit and a fructose unit are joined; the configuration of this glycosidic linkage is α-for glucose and β-for fructose (figure).


Structure of sucrose

Figure- Structure of sucrose

Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase.

An overview of properties of sucrose

  • Sucrose has no free reactive group because the anomeric carbons of both monosaccharides units are involved in the glycosidic bond. Therefore, sucrose neither shows reducing nor mutarotation characters.
  • Sucrose is called invert sugar because the optical activity of sucrose (dextrorotatory) is inverted after hydrolysis (by an acid or an enzyme (invertase or sucrase) into an equimolar mixture of its two components glucose (+52.5) and fructose (-92.5) and the optical activity of the mixture becomes levorotatory.  




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