Lysozyme (muramidase) is an enzyme that catalyzes the hydrolysis of specific glycosidic bonds in mucopolysaccharides that constitute the cell wall of gram-positive bacteria. It is an antibacterial defense in the gastrointestinal tract and is secreted by granulocytes, macrophages, Paneth cells, Brunner's Glands, and normal colonic crypt cells. The primary source of fecal lysozyme is intestinal granulocytes.
Lysozyme is a protein with a molecular weight of approximately 15kDa that belongs to the alkaline glycosidase group. It can be detected in all inflammatory infiltrate cells during an acute attack of Crohn‘s disease. Mononuclear cells also secrete lysozyme actively into the bowel lumen to some extent.
Moderate elevations in fecal lysozyme are commonly associated with significant overgrowth of enteropathogens such as yeast or dysbiotic bacteria. Markedly elevated levels of fecal lysozyme have been identified in colonic inflammatory bowel disease (IBD), such as Crohn's disease and ulcerative colitis, as well as other non-IBD gastrointestinal tract diseases with diarrhea, compared to healthy controls.
In Crohn's disease, excess lysozyme may result from the active secretions of macrophages in the lamina propria and monocytic cells in the granulomas.
In ulcerative colitis, it has been postulated that elevations in fecal lysozyme may be secondary to intestinal loss of granulocytes and their secretory granules.
Paneth cell metaplasia, a phenomenon that occurs with various inflammatory conditions in the large intestine, maybe a minor contributor to fecal lysozyme elevations. Paneth cells are part of the intestinal epithelial lining found in the deepest part of the intestinal crypt, which are the crypts of Lieberkohn. Paneth cells contain lysozyme in their secretory granules, and combined with their phagocytic capability, they help to regulate intestinal microbial flora.
Lysozyme helps determine colonic inflammatory activity rather than small bowel disease. Slightly elevated levels of lysozyme may be treated with anti-inflammatory agents or by removing the antagonist, such as enteroinvasive microorganisms or allergens. Moderate to high lysozyme levels may indicate an active inflammatory bowel condition, often requiring further testing, such as colonoscopy. To rule out IBD, check fecal lactoferrin and calprotectin levels (elevated in IBD).
