β-Galactosidase: intestinal enzyme complex involved in the hydrolysis of lactose to glucose and galactose. Commonly called lactase
Leloir pathway: primary pathway for the conversion of galactose to glucose
Galactosemia: results from defects in any of the 3 primary genes involved in conversion of galactose to glucose
A major form of galactose found in these complex biomolecules is N-acetylgalactosamine (GalNAc). GalNAc is not acquired from the diet but is formed from dietary galactose. The major source of galactose in the human diet is from the disaccharide, lactose, found in dairy products. This sugar comprises around 2% to 8% of milk solids. Lactose is a disaccharide of glucose and galactose. Upon consumption of lactose, it is hydrolyzed to glucose and galactose via the action of the intestinal enzyme complex called β-galactosidase (lactase-glycosylceramidase). The enzyme complex is attached to the surface of intestinal brush border cells via a GPI linkage (see Chapter 38). There are 2 enzymatic activities associated with the β-galactosidase complex, one that hydrolyzes the β-glycosidic linkage in lactose (thereby releasing glucose and galactose), while the other activity hydrolyzes the β-glycosidic bond connecting galactose or glucose to ceramide in ingested glycolipids. Galactose is subsequently absorbed by intestinal enterocytes via the action of the same sodium (Na+)-dependent glucose transporter (SGLT1) that is responsible for glucose absorption. Galactose enters the blood from intestinal enterocytes via GLUT2-mediated transport as for glucose and fructose.
Galactose is an essential carbohydrate needed in the formation of glycolipids and glycoproteins that when present on the surfaces of cells enables cell–cell communication, immune recognition, growth factor-receptor interactions involved in signal transduction events, and many other critical cellular processes.
Although glucose is the form of sugar stored as glycogen within cells, galactose is utilized via conversion to glucose, which can then be oxidized in glycolysis or stored as glycogen. Indeed, up to 30% of ingested galactose is incorporated into glycogen. Galactose enters glycolysis by its conversion to glucose-1-phosphate (G1P). This occurs through a series of steps that is referred to as the Leloir pathway, named after Luis Federico Leloir who determined the overall process of galactose utilization. First, the galactose is phosphorylated by galactokinase to yield galactose-1-phosphate. The galactokinase protein is encoded by the GALK1 gene. There is another gene identified as GALK2 that was originally thought to encode a second galactokinase but was subsequently shown to be a GalNAc kinase. Epimerization of galactose-1-phosphate to G1P requires the transfer of UDP from uridine diphosphoglucose (UDP-glucose) catalyzed by galactose-1-phosphate uridyltransferase (GALT). The GALT-catalyzed reaction generates UDP-galactose and G1P. The UDP-galactose is epimerized to UDP-glucose by UDP-galactose-4 epimerase (GALE). The UDP portion is exchanged for phosphate-generating glucose-1-phosphate, which then is converted to G6P by phosphoglucose mutase. GALE catalyzes 2 distinct but analogous epimerization reactions, the epimerization of UDP-galactose to UDP-glucose and the epimerization of UDP-N-acetylgalactosamine to UDP-N-acetylglucosamine (Figure 12-1).
FIGURE 12-1: Pathway of conversion of (A) galactose to glucose in the liver. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
There are additional pathways of galactose metabolism in humans that do not involve all 3 of the enzymes of the classical Leloir pathway. Galactose can be converted to UDP-glucose by the sequential activities of GALK, UDP-glucose pyrophosphorylase (UGP2), and GALE. Galactose can also be reduced to galactitol by NADPH-dependent aldose reductase. This latter reaction becomes significant in the context of GALT and GALK1 deficiencies that result in galactosemias (Clinical Box 12-1). Finally, galactose can be oxidized to galactonate by galactose dehydrogenase. Under normal conditions, these alternative pathways are responsible for the metabolism of only trace quantities of galactose.
Type 1 galactosemia (classic galactosemia) results from defects in the galactose-1-phosphate uridyltransferase (GALT) gene that leads to severe reductions in enzyme activity. The disease occurs with a frequency between 1:30,000 and 1:60,000 live births. Over 230 different mutations have been described in the gene encoding human GALT, resulting in type 1 galactosemia. The most commonly detected mutation in Caucasians results in the Q188R allele defined by the substitution of arginine (R) for glutamine (Q) at amino acid 188, which lies close to the active site of the enzyme. Homozygotes for the Q188R allele show very little to no GALT activity in their erythrocytes. The majority of heterozygotes are found to have no GALT activity, while others exhibit a low level of enzyme activity generally no more than 20% of the wild type. Whereas the Q188R allele is most common in Caucasians, the most commonly detected mutations in European populations are K285N, S135L, and N314D. The K285N mutation is associated with 0% and 50% GALT activity in homozygous and heterozygous individuals, respectively. Classic galactosemia most often presents within the first weeks after birth and manifests by a failure of neonates to thrive. Vomiting and diarrhea occur following ingestion of milk; hence individuals are termed lactose intolerant. Clinical findings of these disorders include impaired liver function (which if left untreated leads to severe cirrhosis), elevated blood galactose, hypergalactosemia, hyperchloremic metabolic acidosis, urinary galactitol excretion, and hyperaminoaciduria. Upon physical examination infants will be jaundiced and have hepatomegaly. Due to the involvement of the liver, patients will exhibit prolonged bleeding after venous or arterial sampling or will show excessive bruising. Unless controlled by exclusion of galactose from the diet, these galactosemias can go on to produce blindness and fatal liver damage. Blindness is due to the conversion of circulating galactose to the sugar alcohol galactitol, by an NADPH-dependent aldose 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 galactitol 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. Even on a galactose-restricted diet, GALT-deficient individuals exhibit urinary galactitol excretion and persistently elevated erythrocyte galactose-1-phosphate levels. In addition, even with life long restriction of dietary galactose, many patients with classic galactosemia go on to develop serious long-term complications. These long-term complications include ovarian failure in female patients, cognitive impairment, and ataxic neurological disease.