More Information on DNA Digestion

The gastrointestinal tract is constantly exposed to DNA that is released from partially or completely digested foods or feeds, ingested microbes, and DNA from intestinal microflora. Ingested food is mechanically disrupted and the released DNA is cleaved through acid hydrolysis and enzymatic digestion (especially by DNase I from salivary and pancreatic secretions) into small DNA fragments and eventually converted to single nucleotides (McAllan, 1982). The presence of various phosphatases and deaminases continue to destroy the structural integrity of any free DNA. One study with beef steers showed that plant DNA in feed is progressively degraded as it moves through their digestive tract, with over 50% being degraded in the first third of the intestine and 80% having disappeared by the time the digesta reaches the terminal ileum (McAllen, 1980). DNA given directly to steers was shown to be completely degraded into mononucleotides by the animal’s digestive tract in about 4 hours (McAllen, 1982). The generated nucleotides are readily abundant in food and feed and exceed nutritional requirements of the host (Yu, 1998) and gut bacteria (McAllen, 1982). The breakdown products of DNA are absorbed for use in cellular synthetic processes as they can be found in blood and tissues (McAllen, 1982); however, as intact nucleotides they are non-essential nutrients. The nucleotides are typically deaminated before being rapidly absorbed. Once absorbed, they are further catabolized into nitrogenous bases, free bases and other metabolites including sugars and phosphates that are used in cellular biosynthetic pathways (Sonoda and Tatibana, 1978). Interestingly, intestinal epithelial cells have unique salvage pathways for using free nucleotides, owing to their high rate of cell turnover (He and Walker, 1994). Any small polynucleotide DNA fragments that might enter the body would be phagocytized by mononuclear leukocytes and further degraded by cellular enzymes and nucleases (Doerfler, 2000).