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Galactose Metabolism- Solved Subjective Questions
Answer- Galactose is derived from intestinal hydrolysis of the disaccharide lactose, the sugar of milk. It is readily converted in the liver to glucose.
Figure 1- showing the metabolism of galactose. Classical galactosemia occurs due to deficiency of Galactose-1-P uridyl transferase as shown by the block
Metabolism of Galactose
1) Step 1- Formation of galactose-1-P – Galactokinase catalyses the phosphorylation of galactose, using ATP as phosphate donor.(Figure 1)
2) Step 2 -Epimerization of galactose-1-phosphate to Glucose-1-P (G1P) requires the transfer of UDP from uridine diphosphoglucose (UDP-glucose) catalyzed by galactose-1-phosphate uridyl transferase. This generates UDP-galactose and G1P.
3) Step 3- Epimerization of UDP galactose to UDP Glucose- The UDP-galactose is epimerized to UDP-glucose by UDP-galactose-4 epimerase. The reaction involves oxidation, then reduction, at carbon 4, with NAD+ as coenzyme.
Fate of UDP glucose- i) The UDPGlc can be incorporated into glycogen.
ii) Since the epimerase reaction is freely reversible, UDP glucose can be converted to UDP galactose, so that galactose is not a dietary essential.
iii) UDPGlc can also be used in the Uronic acid pathway for the formation of UDP glucuronic acid which can be utilized for the conjugation reactions.
Fate of glucose-1-P- Glucose-1-phosphate can be converted to G6P by phosphoglucose mutase. Glucose -6-P can then enter glycolysis (Figure 2) or can be converted to free glucose by the action of glucose-6 phosphatase.
Figure-2 showing the entry of galactose in to glycolytic pathway
Fate of UDP galactose
UDP Galactose supplies galactose moiety for the formation of lactose, glycolipids (Cerebrosides), Proteoglycans, and glycoproteins.
Biosynthesis of Galacitol
Galacitol is an alcohol that is made from galactose. The pathway is similar to that of formation of glucose. Because of the excessive quantity of galactose, some of it is converted to galacitol, which build up in the body and is excessively excreted in urine. Galacitol appears to be toxic at high concentration and produces tissue injuries, especially cataract formation.(see the details in galactosemia )
Q.2- A patient complains of gastric discomfort upon consuming milk. The patient also shows signs of liver, kidney and brain dysfunction. This dysfunction is due to the build up of the toxic compound, galactose-1-P.
- What enzyme is the patient lacking?
- What reaction is not being catalyzed?
- What kind of laboratory investigations should be carried out for the confirmation of diagnosis?
Suggest a possible recommendation for the patient to follow in order to avoid further tissue damage.
Answer- The patient might be suffering from Classical Galactosemia as is evident from the signs and symptoms and the accumulation of Galactose-1-P.
Galactose-1-phosphate uridyl Transferase (GALT) deficiency is the most common enzyme deficiency that causes Galactosemia.
Galactose-1-phosphate uridyl Transferase (GALT) catalyzes conversion of galactose-1-p to UDP Galactose. This is an important reaction during the metabolism of galactose,
Galactosemia is associated with the following 3 enzyme deficiencies;
A) Classical galactosemia is a major symptom of two enzyme defects. It results from loss of the enzyme galactose-1-phosphate uridyl transferase.
B) The second form of galactosemia results from a loss of galactokinase.
Clinical manifestations- These two defects are manifested by-
1) A failure of neonates to thrive.
2)Vomiting and diarrhea occur following ingestion of milk; hence individuals are termed lactose intolerant.
3)Impaired liver function (which if left untreated leads to severe cirrhosis),
4) Elevated blood galactose, hypergalactosemia, hyperchloremic metabolic acidosis, urinary galacitol excretion and hyper aminoaciduria. Unless controlled by exclusion of galactose from the diet, these galactosemias can go on to produce blindness and fatal liver damage. Even on a galactose-restricted diet, transferase-deficient individuals exhibit urinary galacitol excretion and persistently elevated erythrocyte galactose-1-phosphate levels. B
5) Blindness is due to the conversion of circulating galactose to the sugar alcohol galacitol, by an NADPH-dependent galactose reductase that is present in neural tissue and in the lens of the eye. At normal circulating levels of galactose this enzyme activity causes no pathological effects. However, a high concentration of galacitol in the lens causes osmotic swelling, with the resultant formation of cataracts and other symptoms. The principal treatment of these disorders is to eliminate lactose from the diet.
6) Hypoglycemia is a very common finding due to inhibition of Phosphoglucomutase enzyme.
7)Mental retardation occurs in the untreated cases due to accumulation of galactose and galactose-1-P.
C) The third disorder results from a deficiency of UDP-galactose-4-epimerase-Two different forms of this deficiency have been found. One is benign affecting only red and white blood cells. The other affects multiple tissues and manifests symptoms similar to the transferase deficiency. Treatment involves restriction of dietary galactose.
I) Urine test for reducing sugar is positive. The confirmation for the presence of galactose can be done by Thin Layer Chromatography.
II) Galactose Tolerance Test- A galactose Tolerance Test is abnormal in these patients. There is much higher than normal rise in blood galactose after the administration of galactose, and the elevation persists for a much longer time than normal. The galactose Tolerance Test in a normal person shows the level in the blood rising to a maximum at about half an hour after the galactose administration. It then returns to base line value within 1 hour.
III) Serum Transaminases- Both the serum Transaminases are elevated, suggesting the possibility of liver damage.
IV) Serum Bilirubin- is elevated. A high proportion of the bilirubin is unconjugated.
V) Estimation of Galactose–1-phosphate Uridyl Transferase- Finally the diagnosis can be made by demonstrating that the galatose-1-phosphate Transferase is absent in the erythrocytes.
Treatment- Galactosemic children are to be kept on a galactose free diet. Synthetic diets or soya milk can be substituted.
Q.3- Explain why a galactosemic child maintained on a galactose free diet manifests normal growth and development? What is the alternative pathway for the formation of galactose in the body?
Answer- Galactose is required in the brain for the synthesis of glycolipids like Cerebrosides and gangliosides. It is essential for the synthesis of glycoproteins like membrane proteins and protein hormones.
Galactosemia is treated by giving a galactose free diet. In the absence of dietary galactose the patient can thrive, an alternative source of galactose exists even if the patient is fed on a galactose free diet because the much needed galactose for the synthesis of biomolecules comes from UDP glucose by the action of Epimerase enzyme (Figure 1). First, glucose is converted to glucose-1-phosphate and then to UDP- glucose. Next an Epimerase converts the UDP- glucose to UDP- galactose. Hence a child on a galactose free diet can have a normal growth and development at the cost of glucose compensating for galactose.
Patients with Transferase deficiency, eventually have a less severe disease because an alternative enzyme for galactose utilization develops during childhood. This enzyme enables the galactose-1-phosphate to bypass the uridyl Transferase step. Instead of reacting with UDP –glucose, the galactose-1-phosphate reacts with UTP to form UDP galactose. The alternative pathway can handle only a fraction of the galactose that the uridyl Transferase reaction can. It handles enough, however, to enable the person to have some galactose in the diet. Therefore a patient does not have to maintain as severely a restrictive diet as he or she grows older. If the Galactokinase were deficient alternate pathway could not function later in life because the patient could not produce galactose -1 phosphate in the diet. Therefore the patient would have to remain on a much more restrictive diet throughout life. Fortunately very few cases of galactosemia are caused by Galactokinase deficiency.
Q.4- A pregnant woman who was extremely lactose intolerant asked her physician if she could still be able to breast feed her baby even though she could not drink milk or dairy products. What advice should be given?
Answer- The patient should be advised to breast feed the baby after delivery. Lactose is synthesized from UDP galactose and glucose; however galactose is not required in the diet for lactose synthesis, because galactose can be synthesized from glucose.
Lactose is unique in the sense that it can only be synthesized in the mammary gland of the adult female for short period during lactation. Lactose synthase present in the endoplasmic reticulum of the lactating mammary gland, catalyze the last step in the lactose biosynthesis, the transfer of galactose from UDP galactose to glucose.
Figure 3- showing the formation of lactose, dietary galactose is not needed for lactose synthesis. Glucose is converted first to Glucose-6-p, then to Glucose-1-p, which is converted to UDP glucose and then to UDP galactose to condense with glucose to form lactose.
Lactose synthase attaches the anomeric carbon of galactose to C4 alcohol group of glucose to form a glycosidic bond. Lactose synthase is composed of a galactosyl transferase and α lactalbumin which is a regulatory subunit. Lactose synthase is activated by Prolactin hormone. In the non lactating mammary glands, this enzyme is inactive, so lactose is not synthesized and UDP galactose is alternatively used for the formation of glycolipids or glycoproteins.
Q.5- High concentration of galactose-1-phosphate inhibits Phosphoglucomutase, the enzyme that converts glucose-6-P to Glucose-1-P. How can this inhibition account for hypoglycemia and jaundice that accompany galactose-1-P uridyl transferase deficiency?
Answer- Inhibition of Phosphoglucomutase by galactose-1-P results in hypoglycemia due to interference in formation of UDP glucose (Glycogen precursor) and also in the degradation of glycogen back to glucose-6-p.
90% of glycogen is converted to Glucose-1-p which is converted to glucose-6-p by Phosphoglucomutase enzyme. When Phosphoglucomutase is inhibited less glucose-6-p is formed and hence less free glucose is formed to be exported. Thus stored glycogen is only 10% efficient in raising blood glucose level and hence hypoglycemia results.
UDP glucose levels are reduced, because glucose-1-p is required for the formation of UDP glucose. Hence in the absence of Phosphoglucomutase activity, glucose-6-p (derived from the activity of glucokinase or from gluconeogenesis), can not be converted to glucose-1-p. This prevents the formation of UDP glucuronic acid which is required to convert bilirubin to bilirubin diglucuronide form for transport in to bile. Bilirubin accumulates in tissues causing jaundice.
Besides conjugation, the uptake of bilirubin in hepatocytes is also affected resulting in unconjugated type of hyperbilirubunemia.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!