Metabolism and Transport. Galactose is a hexose monosaccharide whose dietary intake is usually in the form of the disaccharide lactose (milk sugar). Lactose is split by the digestive enzyme lactase into its hexose moities, glucose and galactose. Galactose shares the same transport mechanisms as glucose in the enterocytes, namely apical SGLT cotransporters and the basolateral GLUT 2. It enters the portal blood and is practically cleared in its passage through the liver, so that little or no galactose is seen in the systemic blood above 1 mmol/L, even after ingesting as much as 100 g of lactose. Ingestion of galactose without glucose, however, induces higher plasma concentrations. Alcohol is said to depress galactose uptake and metabolism by the liver, leading to an increased level in the blood (galactosemia). In the liver cells, it is converted by the enzyme galactokinase into galactose-1-phosphate. This, in turn, is converted by a two-stage enzymic transformation into glucose-1-phosphate, which is converted into glycogen. Although in theory glucose-1-phosphate can enter the glycolytic pathway, it does not normally do so to any great extent. Most tissues have enzymes that can metabolize galactose. However, even in the complete absence of dietary galactose, glucose can be converted into galactose and supply cellular needs for galactose if required. Many structural elements of cells and tissues (viz., glycoproteins and mucopolysaccharides) contain galactose.
Cataracts and Inborn Errors. Galactose levels in peripheral blood normally do not go above 1 mmol/L. If they do (galactosemia), then various tissues can remove it from the blood and convert it into galactitol (dulcitol) by the enzyme aldehyde reductase. As it is nonmetabolized, it builds up in the tissues and causes pathologic changes because of the high osmotic pressure created. In the lens of the eye, such a scenario causes cataracts ( 46). Cataracts can also occur in two inborn errors of galactose metabolism caused by deficiencies of the enzymes galactose-1-phosphate uridyltransferase and galactokinase. The former enzyme deficiency creates classical galactosemia. Unless it is treated promptly in the neonate by withdrawing galactose from the diet (taken in from the lactose content of normal milk), either death or severe mental retardation can occur. Cataracts can also occur as a complication of diabetes mellitus, in which blood glucose is raised to high levels.
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