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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