Lactose is hydrolyzed in the intestine by the enzyme lactase-phlorizin hydrolase to glucose and galactose. In humans, D-glucose and D-galactose are the only nutritionally significant monosaccharides that are actively absorbed. Although glucose and galactose can cross the intestinal mucosa down a diffusion gradient, the slowness of this method is such that water would diffuse in the opposite direction leading to a lessening in the concentration gradient. Thus, a rapid transport mechanism exists for glucose and galactose, particularly in infants. The 'coupled carrier' hypothesis is generally accepted as the main mechanism. In the small intestine and proximal tubule of the kidney, D-glucose is absorbed by epithelial cells via a sodium-dependent cotransport system existing at the luminal membrane level and a sodium-independent transport system at the basolat-eral membrane level. It is suggested that the potential difference across the brush border membrane of the cell also plays an important role in the mechanism which concentrates sugar in the cell.
The genetic functional defects of this cotransport system are expressed in two main clinical entities; selective congenital glucose and galactose malabsorption by the intestine discussed below, and familial renal glycosuria. Once galactose is absorbed, it must be converted to glucose for utilization. This occurs primarily by the pathways explained below. Three distinct enzymatic defects are responsible for the conditions generally described as galactosemia.
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