Galactose Explained

Galactose (galacto- + -ose, "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose.[1] It is an aldohexose and a C-4 epimer of glucose.[2] A galactose molecule linked with a glucose molecule forms a lactose molecule.

Galactan is a polymeric form of galactose found in hemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates.[3]

D-Galactose is also known as brain sugar since it is a component of glycoproteins (oligosaccharide-protein compounds) found in nerve tissue.[4]

Etymology

The word galactose was coined by Charles Weissman[5] in the mid-19th century and is derived from Greek γαλακτος, galaktos, (of milk) and the generic chemical suffix for sugars -ose.[6] The etymology is comparable to that of the word lactose in that both contain roots meaning "milk sugar". Lactose is a disaccharide of galactose plus glucose.

Structure and isomerism

Galactose exists in both open-chain and cyclic form. The open-chain form has a carbonyl at the end of the chain.

Four isomers are cyclic, two of them with a pyranose (six-membered) ring, two with a furanose (five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa,[7] [8] and is recognized by a putative chordate immune lectin intelectin through its exocyclic 1,2-diol. In the cyclic form there are two anomers, named alpha and beta, since the transition from the open-chain form to the cyclic form involves the creation of a new stereocenter at the site of the open-chain carbonyl.[9]

The IR spectra for galactose shows a broad, strong stretch from roughly wavenumber 2500 cm−1 to wavenumber 3700 cm−1.[10]

The Proton NMR spectra for galactose includes peaks at 4.7 ppm (D2O), 4.15 ppm (−CH2OH), 3.75, 3.61, 3.48 and 3.20 ppm (−CH2 of ring), 2.79–1.90 ppm (−OH).

Relationship to lactose

Galactose is a monosaccharide. When combined with glucose (another monosaccharide) through a condensation reaction, the result is a disaccharide called lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzymes lactase and β-galactosidase. The latter is produced by the lac operon in Escherichia coli.

In nature, lactose is found primarily in milk and milk products. Consequently, various food products made with dairy-derived ingredients can contain lactose.[11] Galactose metabolism, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway. The enzymes are listed in the order of the metabolic pathway: galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4’-epimerase (GALE).

In human lactation, galactose is required in a 1 to 1 ratio with glucose to enable the mammary glands to synthesize and secrete lactose. In a study where women were fed a diet containing galactose, 69 ± 6% of glucose and 54 ± 4% of galactose in the lactose they produced were derived directly from plasma glucose, while 7 ± 2% of the glucose and 12 ± 2% of the galactose in the lactose, were derived directly from plasma galactose. 25 ± 8% of the glucose and 35 ± 6% of the galactose was synthesized from smaller molecules such as glycerol or acetate in a process referred to in the paper as hexoneogenesis. This suggests that the synthesis of galactose is supplemented by direct uptake and of use of plasma galactose when present.[12]

Metabolism

Glucose is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates, molecules with at least one sugar attached to a protein or lipid. Many speculate that it is for this reason that a pathway for rapid conversion from galactose to glucose has been highly conserved among many species.[13]

The main pathway of galactose metabolism is the Leloir pathway; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff Pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principal enzymes: Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P; Galactose-1-phosphate uridyltransferase (GALT) transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose; and finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway.[14]

The above mechanisms for galactose metabolism are necessary because the human body cannot directly convert galactose into energy, and must first go through one of these processes in order to utilize the sugar.[15]

Galactosemia is an inability to properly break down galactose due to a genetically inherited mutation in one of the enzymes in the Leloir pathway. As a result, the consumption of even small quantities is harmful to galactosemics.[16]

Sources

Galactose is found in dairy products, avocados, sugar beets, other gums and mucilages. It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues; and is a by-product from the third-generation ethanol production process (from macroalgae).

Clinical significance

Chronic systemic exposure of mice, rats, and Drosophila to D-galactose causes the acceleration of senescence (aging). It has been reported that high dose exposure of D-galactose (120 mg/kg) can cause reduced sperm concentration and sperm motility in rodents and has been extensively used as an aging model when administered subcutaneously.[17] [18] [19] Two studies have suggested a possible link between galactose in milk and ovarian cancer.[20] [21] Other studies show no correlation, even in the presence of defective galactose metabolism.[22] [23] More recently, pooled analysis done by the Harvard School of Public Health showed no specific correlation between lactose-containing foods and ovarian cancer, and showed statistically insignificant increases in risk for consumption of lactose at 30 g/day.[24] More research is necessary to ascertain possible risks.

Some ongoing studies suggest galactose may have a role in treatment of focal segmental glomerulosclerosis (a kidney disease resulting in kidney failure and proteinuria).[25] This effect is likely to be a result of binding of galactose to FSGS factor.[26]

Galactose is a component of the antigens (chemical markers) present on blood cells that distinguish blood type within the ABO blood group system. In O and A antigens, there are two monomers of galactose on the antigens, whereas in the B antigens there are three monomers of galactose.[27]

A disaccharide composed of two units of galactose, galactose-alpha-1,3-galactose (alpha-gal), has been recognized as a potential allergen present in mammal meat. Alpha-gal allergy may be triggered by lone star tick bites.[28]

Galactose in sodium saccharin solution has also been found to cause conditioned flavor avoidance in adult female rats within a laboratory setting when combined with intragastric injections.[29] The reason for this flavor avoidance is still unknown, however it is possible that a decrease in the levels of the enzymes required to convert galactose to glucose in the liver of the rats could be responsible.

History

In 1855, E. O. Erdmann noted that hydrolysis of lactose produced a substance besides glucose.[30] [31]

Galactose was first isolated and studied by Louis Pasteur in 1856 and he called it "lactose".[32] In 1860, Berthelot renamed it "galactose" or "glucose lactique".[33] [34] In 1894, Emil Fischer and Robert Morrell determined the configuration of galactose.[35]

See also

Notes and References

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  2. Book: Organic Reactions Stereochemistry And Mechanism (Through Solved Problems) . Kalsi PS . 2007 . New Age International . 9788122417661 . 43 . en.
  3. Book: The Antibodies . Zanetti . Maurizio . Capra . Donald J. . vanc . 2003-09-02 . CRC Press . 9780203216514 . 78 .
  4. Web site: 16.3 Important Hexoses The Basics of General, Organic, and Biological Chemistry . 2022-05-06 . courses.lumenlearning.com.
  5. Web site: Charles Weismann in the 1940 Census . Ancestry. 26 December 2017. en.
  6. Book: Bhat. Paike Jayadeva . vanc . Galactose Regulon of Yeast: From Genetics to Systems Biology . Springer Science & Business Media. 26 December 2017. en. 2 March 2008. 9783540740155.
  7. Nassau PM, Martin SL, Brown RE, Weston A, Monsey D, McNeil MR, Duncan K . 6 . Galactofuranose biosynthesis in Escherichia coli K-12: identification and cloning of UDP-galactopyranose mutase . Journal of Bacteriology . 178 . 4 . 1047–52 . February 1996 . 8576037 . 177764 . 10.1128/jb.178.4.1047-1052.1996.
  8. Tefsen B, Ram AF, van Die I, Routier FH . Galactofuranose in eukaryotes: aspects of biosynthesis and functional impact . Glycobiology . 22 . 4 . 456–69 . April 2012 . 21940757 . 10.1093/glycob/cwr144 . free .
  9. Web site: Ophardt, C. Galactose . 2015-11-26 . https://web.archive.org/web/20060908164848/http://elmhcx9.elmhurst.edu/~chm/vchembook/543galactose.html . 2006-09-08 . dead .
  10. Tunki. Lakshmi. Kulhari. Hitesh. Vadithe. Lakshma Nayak. Kuncha. Madhusudana. Bhargava. Suresh. Pooja. Deep. Sistla. Ramakrishna. 2019-09-01. Modulating the site-specific oral delivery of sorafenib using sugar-grafted nanoparticles for hepatocellular carcinoma treatment. European Journal of Pharmaceutical Sciences. en. 137. 104978. 10.1016/j.ejps.2019.104978. 31254645. 195764874. 0928-0987.
  11. Web site: Lactose Intolerance – National Digestive Diseases Information Clearinghouse . Staff . digestive.niddk.nih.gov . June 2009 . January 11, 2014 . https://web.archive.org/web/20111125192619/http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/ . November 25, 2011 . dead .
  12. Sunehag A, Tigas S, Haymond MW . Contribution of plasma galactose and glucose to milk lactose synthesis during galactose ingestion . The Journal of Clinical Endocrinology and Metabolism . 88 . 1 . 225–9 . January 2003 . 12519857 . 10.1210/jc.2002-020768 . free .
  13. Book: https://ommbid.mhmedical.com/content.aspx?bookid=971&sectionid=62672411 . subscription . The Online Metabolic and Molecular Bases of Inherited Disease . Galactosemia . Fridovich-Keil JL, Walter JH . Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson KM, Mitchell G . 2018-06-25 . 2018-06-26 . https://web.archive.org/web/20180626030220/https://ommbid.mhmedical.com/content.aspx?bookid=971&sectionid=62672411 . dead .
    a 4 b 21 c 22 d 22
  14. Bosch AM . Classical galactosaemia revisited . Journal of Inherited Metabolic Disease . 29 . 4 . 516–25 . August 2006 . 16838075 . 10.1007/s10545-006-0382-0 . 16382462 .
    a 517 b 516 c 519
  15. Book: Berg. Jeremy M.. Tymoczko. John L.. Stryer. Lubert. 2013. Stryer Biochemie. 10.1007/978-3-8274-2989-6. 978-3-8274-2988-9.
  16. Classic Galactosemia and Clinical Variant Galactosemia. Gerard T . Berry . vanc . Nih.gov. 17 May 2015. University of Washington, Seattle. 1993. 20301691.
  17. Pourmemar E, Majdi A, Haramshahi M, Talebi M, Karimi P, Sadigh-Eteghad S . Intranasal Cerebrolysin Attenuates Learning and Memory Impairments in D-galactose-Induced Senescence in Mice . Experimental Gerontology . 87 . Pt A . 16–22 . January 2017 . 27894939 . 10.1016/j.exger.2016.11.011 . 40793896 .
  18. Cui X, Zuo P, Zhang Q, Li X, Hu Y, Long J, Packer L, Liu J . Chronic systemic D-galactose exposure induces memory loss, neurodegeneration, and oxidative damage in mice: protective effects of R-alpha-lipoic acid . Journal of Neuroscience Research . 84 . 3 . 647–54 . August 2006 . 16710848 . 10.1002/jnr.20899 . 13641006 .
  19. Zhou YY, Ji XF, Fu JP, Zhu XJ, Li RH, Mu CK, Wang CL, Song WW . 6 . Gene Transcriptional and Metabolic Profile Changes in Mimetic Aging Mice Induced by D-Galactose . PLOS ONE . 10 . 7 . e0132088 . 2015-07-15 . 26176541 . 4503422 . 10.1371/journal.pone.0132088 . 2015PLoSO..1032088Z . free .
  20. Cramer DW . Lactase persistence and milk consumption as determinants of ovarian cancer risk . American Journal of Epidemiology . 130 . 5 . 904–10 . November 1989 . 2510499 . 10.1093/oxfordjournals.aje.a115423.
  21. Cramer DW, Harlow BL, Willett WC, Welch WR, Bell DA, Scully RE, Ng WG, Knapp RC . Galactose consumption and metabolism in relation to the risk of ovarian cancer . Lancet . 2 . 8654 . 66–71 . July 1989 . 2567871 . 10.1016/S0140-6736(89)90313-9 . 34304536 .
  22. Goodman MT, Wu AH, Tung KH, McDuffie K, Cramer DW, Wilkens LR, Terada K, Reichardt JK, Ng WG . 6 . Association of galactose-1-phosphate uridyltransferase activity and N314D genotype with the risk of ovarian cancer . American Journal of Epidemiology . 156 . 8 . 693–701 . October 2002 . 12370157 . 10.1093/aje/kwf104 . free .
  23. Fung WL, Risch H, McLaughlin J, Rosen B, Cole D, Vesprini D, Narod SA . The N314D polymorphism of galactose-1-phosphate uridyl transferase does not modify the risk of ovarian cancer . Cancer Epidemiology, Biomarkers & Prevention . 12 . 7 . 678–80 . July 2003 . 12869412 .
  24. Genkinger JM, Hunter DJ, Spiegelman D, Anderson KE, Arslan A, Beeson WL, Buring JE, Fraser GE, Freudenheim JL, Goldbohm RA, Hankinson SE, Jacobs DR, Koushik A, Lacey JV, Larsson SC, Leitzmann M, McCullough ML, Miller AB, Rodriguez C, Rohan TE, Schouten LJ, Shore R, Smit E, Wolk A, Zhang SM, Smith-Warner SA . 6 . Dairy products and ovarian cancer: a pooled analysis of 12 cohort studies . Cancer Epidemiology, Biomarkers & Prevention . 15 . 2 . 364–72 . February 2006 . 16492930 . 10.1158/1055-9965.EPI-05-0484 .
  25. De Smet E, Rioux JP, Ammann H, Déziel C, Quérin S . FSGS permeability factor-associated nephrotic syndrome: remission after oral galactose therapy . Nephrology, Dialysis, Transplantation . 24 . 9 . 2938–40 . September 2009 . 19509024 . 10.1093/ndt/gfp278 . free .
  26. McCarthy ET, Sharma M, Savin VJ . Circulating permeability factors in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis . Clinical Journal of the American Society of Nephrology . 5 . 11 . 2115–21 . November 2010 . 20966123 . 10.2215/CJN.03800609 . free .
  27. Book: Understanding Biology. 3rd. Peter H. . Raven . George B. . Johnson . 203 . 978-0-697-22213-8 . 1995 . Mills CJ . WM C. Brown.
  28. Web site: Alpha-gal syndrome - Symptoms and causes. 2022-02-25. Mayo Clinic. en.
  29. Sclafani. Anthony. Fanizza. Lawrence J. Azzara. Anthony V. 1999-08-15. Conditioned Flavor Avoidance, Preference, and Indifference Produced by Intragastric Infusions of Galactose, Glucose, and Fructose in Rats. Physiology & Behavior. en. 67. 2. 227–234. 10.1016/S0031-9384(99)00053-0. 10477054. 37225025. 0031-9384.
  30. Eduard Otto . Erdmann . vanc . 1855 . Dissertatio de saccharo lactico et amylaceo . Dissertation on milk sugar and starch . la . University of Berlin .
  31. Web site: Jahresbericht über die Fortschritte der reinen, pharmaceutischen und technischen Chemie . Annual report on progress in pure, pharmaceutical, and technical chemistry . de . 1855 . 671–673 . see especially p. 673.
  32. Pasteur L . 1856 . Note sur le sucre de lait . Note on milk sugar . fr . Comptes rendus . 42 . 347–351 . From page 348: Je propose de le nommer lactose. (I propose to name it lactose.).
  33. Marcellin . Berthelot . vanc . Chimie organique fondée sur la synthèse . Organic chemistry based on synthesis . fr . Paris, France . Mallet-Bachelier . 1860 . 2 . 248–249 .
  34. "Galactose" — from the Ancient Greek γάλακτος (gálaktos, “milk”).
  35. Emil . Fischer . Robert S. . Morrell . vanc . 1894 . Ueber die Configuration der Rhamnose und Galactose . On the configuration of rhamnose and galactose . de . Berichte der Deutschen Chemischen Gesellschaft zu Berlin . 27 . 382–394 . 10.1002/cber.18940270177. The configuration of galactose appears on page 385.

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