Galactose (/ɡəˈlæktoʊs/,galacto- +-ose, sometimes abbreviatedGal, is a commonmonosaccharide, i.e. a simple sugar. It is classified as a reducing hexose, more specifically analdohexose.[3] In terms of structure, it is a C-4epimer of glucose. A white, water-soluble solid, it is about assweet asglucose, and about 65% as sweet assucrose.[4]
Galactan is apolymeric form of galactose found inhemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates.[5]
D-Galactose is also known as brain sugar since it is a component ofglycoproteins (oligosaccharide-protein compounds) found innerve tissue.[6]Galactofuranose occurs in bacteria, fungi and protozoa,[7][8] and is recognized by a putative chordate immune lectinintelectin through its exocyclic 1,2-diol.
In humanlactation, galactose is required in a 1 to 1 ratio with glucose to enable themammary 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 fromplasma 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 in a process referred to in the paper ashexoneogenesis. This suggests that the synthesis of galactose is supplemented by direct uptake and of use of plasma galactose when present.[11]
Cyclic forms of galactoseChair conformation of D-galactopyranose
Galactose exists in both open-chain and cyclic form. The open-chain form is analdehyde (RCHO).
Four isomers are cyclic, two of them with apyranose (six-membered) ring and two with afuranose (five-membered) ring. Each cyclic form can exist as twoanomers, named alpha and beta, since a newstereocenter is generated upon cyclization at the site of the carbonyl.[12] In the pyranose form, the OH group on C-3 is axial.
Metabolism of commonmonosaccharides and some biochemical reactions of glucose
Galactose metabolism
Glucose is more stable than galactose and is less susceptible to the formation of nonspecificglycoconjugates, 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 beenhighly conserved among many species.[13]
The main pathway of galactose metabolism is theLeloir pathway; humans and other species, however, have been noted to contain several alternate pathways, such as theDe Ley Doudoroff Pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose toUDP-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 use galactose for energy metabolism, and it must first go through one of these processes.[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]
Chronic systemic exposure ofmice,rats, andDrosophila to D-galactose causes the acceleration ofsenescence (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 andovarian cancer.[20][21] Other studies show no correlation, even in the presence of defective galactose metabolism.[22][23] More recently, pooled analysis done by theHarvard 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.[citation needed]
Some ongoing studies suggest galactose may have a role in treatment offocal 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 theantigens (chemical markers) present on blood cells that distinguish blood type within theABO blood group system. In O and A antigens, there are twomonomers of galactose on the antigens, whereas in the B antigens there are three monomers of galactose.[27]
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, a decrease in the levels of the enzymes required to convert galactose to glucose in the liver of the rats could be responsible.[29]
In 1855, E. O. Erdmann noted that hydrolysis of lactose produced a substance besides glucose.[30][31]
Galactose was first isolated and studied byLouis 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 theconfiguration of galactose.[35]
The wordgalactose is derived fromGreekγάλακτος,galaktos'of milk', and the generic chemical suffix for sugars-ose.[3] The etymology is comparable to that of the wordlactose in that both contain roots meaning "milk sugar".
TheIR spectra for galactose shows a broad, strong stretch from roughly wavenumber 2500 cm−1 to wavenumber 3700 cm−1.[36]
TheProton 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).[36]
^abConte, Federica; Van Buuringen, Nicole; Voermans, Nicol C.; Lefeber, Dirk J. (2021). "Galactose in human metabolism, glycosylation and congenital metabolic diseases: Time for a closer look".Biochimica et Biophysica Acta (BBA) - General Subjects.1865 (8) 129898.doi:10.1016/j.bbagen.2021.129898.hdl:2066/235637.PMID33878388.
^Fridovich-Keil JL, Walter JH."Galactosemia". In Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson KM, Mitchell G (eds.).The Online Metabolic and Molecular Bases of Inherited Disease. Archived fromthe original on 2018-06-26. Retrieved2018-06-25. a 4 b 21 c 22 d 22
^Pourmemar E, Majdi A, Haramshahi M, Talebi M, Karimi P, Sadigh-Eteghad S (January 2017). "Intranasal Cerebrolysin Attenuates Learning and Memory Impairments in D-galactose-Induced Senescence in Mice".Experimental Gerontology.87 (Pt A):16–22.doi:10.1016/j.exger.2016.11.011.PMID27894939.S2CID40793896.
^Cui X, Zuo P, Zhang Q, Li X, Hu Y, Long J, Packer L, Liu J (August 2006). "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.doi:10.1002/jnr.20899.PMID16710848.S2CID13641006.
^Cramer DW (November 1989). "Lactase persistence and milk consumption as determinants of ovarian cancer risk".American Journal of Epidemiology.130 (5):904–10.doi:10.1093/oxfordjournals.aje.a115423.PMID2510499.
^Cramer DW, Harlow BL, Willett WC, Welch WR, Bell DA, Scully RE, Ng WG, Knapp RC (July 1989). "Galactose consumption and metabolism in relation to the risk of ovarian cancer".Lancet.2 (8654):66–71.doi:10.1016/S0140-6736(89)90313-9.PMID2567871.S2CID34304536.
^Fung WL, Risch H, McLaughlin J, Rosen B, Cole D, Vesprini D, Narod SA (July 2003). "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.PMID12869412.
^Genkinger JM, Hunter DJ, Spiegelman D, Anderson KE, Arslan A, Beeson WL, et al. (February 2006). "Dairy products and ovarian cancer: a pooled analysis of 12 cohort studies".Cancer Epidemiology, Biomarkers & Prevention.15 (2):364–72.doi:10.1158/1055-9965.EPI-05-0484.PMID16492930.
^Pasteur L (1856)."Note sur le sucre de lait" [Note on milk sugar].Comptes rendus (in French).42:347–351.From page 348: Je propose de le nommerlactose. (I propose to name itlactose.)
