Nucleic acid digestion - brush border enzymes, end products & transport mechanism

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• Further digestion occurs at the microvilli (brush border) of the epithelial cells of the villi in the small intestine. • Two brush border enzymes complete nucleic acid digestion: • Phosphatases, which catalyze the cleavage of a phosphate to form a nucleoside (nitrogenous base and pentose sugar). • Nucleosidases, which catalyze the breaking of the covalent bond that holds the nitrogenous base to the pentose sugar. • The final end products of nucleic acid digestion are: • nitrogenous oases. • Pentose sugars. • Phosphate ions. • Absorption of nucleic acid mainly occurs in the duodenum and jejunum of the small intestine. • At the intestinal villus, all nucleic acids are absorbed as: • Nitrogenous bases. • Pentose sugars. • Phosphate ions. • Membrane transport proteins carry the products of nucleotide digestion into epithelial cells from the lumen. • Some involve active transport; some involve secondary active transport. • Through diffusion, the products of nucleotide digestion are transported from the intestinal epithelial cells: • Across the basolateral membrane. • Into the interstitial fluid. • And, finally, into the blood. • The nucleotide digestion products are transported by blood circulation to the liver and other tissues where they undergo further degradation.

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