Lactase-phlorizin hydrolase

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The LCT gene at 2q21.3 codes for the 1927 pro-peptide of lactase-phlorizin hydrolase which hydrolyses lactose in the brush border of the small intestine. It has two enzyme sites:

lactose + H2O => D-galactose + D-glucose
Glycosyl-N-acylsphingosine + H2O = N-acylsphingosine + sugar

Dysfuctional mutations produce autosomal recessive congenital lactase deficiency (COLACD, disaccharide intolerance II, hereditary alactasia). This results in watery diarrhoea in infants fed with breast milk or other lactose-containing formulas.

Normal regulation by a non-coding region of MCM6 results in the LCT gene being switched off in adults, causing a similar syndrome which has been selected against in populations exposed to diary products such as Northern Europeans