The quantification and distinction of particulate guanylyl cyclases in the human intestine were considered by an enzymatic approach, which comprised the signal transduction from receptor binding to cGMP formation, and, in addition, by showing the expression of an intracellular portion of these transmembrane proteins. Basal guanylyl cyclase (GC) activities were 50 to 80 pmol cGMP formation/min/mg protein and were stimulated up to twofold by heat stable enterotoxin, but were not significantly influenced by atrial natriuretic factor. Enzymatic analysis of colonoscopic specimens pointed to the prevalence of guanylyl cyclase C in the terminal ileum and in the large bowel including colon ascendens, colon descendens, sigmoid, and rectum. The availability of sequence information on human guanylyl cyclases permitted the development of a polymerase chain reaction approach for distinguishing the expression of GC-A and GC-C in human tissue samples. The expression levels of particulate guanylyl cyclases found by polymerase chain reaction in surgical biopsy specimens confirmed the enzymatic data, in that substantial expression of GC-C was found not only in the small intestine but also in the large bowel. According to the restriction mapping of amplificates, GC-C prevailed over GC-A throughout the human intestine, particularly in the mucosal layers.
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