Kinetic Evidence for the Presence of Two Postaglandin Receptor Sites Regulating the Activity of Intestinal Adenylate Cyclase Sensitive to Escherichia coli Enterotoxin

Abstract

Kinetic behavior most consistent with the presence of two independent, but simultaneously acting, regulatory effector sites for prostaglandins has been presented for adenylate cyclase (EC 4.6.1.1) of rabbit intestinal epithelial cells. One site regulates activation of the catalytic site, while the other site regulates inhibition. A synthetic prostaglandin analogue, 7-oxa-13-prostynoic acid, is recognized at both sites in a concentration-dependent manner. At concentrations of 7-oxa-13-prostynoic acid less than 45 Ag/ml, activation is seen, while at higher concentrations, inhibition is seen. Different naturally occurring prostaglandins appear to be site-specific. Prostaglandin El gives only activation of the cyclase, while prostaglandin Al gives only inhibition of the activated cyclase. When saturating concentrations of prostaglandin E, are used to activate adenylate cyclase, no further activation by 7-oxa-13-prostynoic acid can be elicited, indicating that both molecules activate at the same site. The similarity of inhibition constants for both 7-oxa-13-prostynoic acid and prostaglandin A, suggests that the mode of binding is the same for both compounds and that they probably inhibit by acting at the same site. The inhibition by 7-oxa-13-prostynoic acid and by prostaglandin Al overrides enzyme activation produced by either Escherichia coli enterotoxin, prostaglandin El, or sodium fluoride, suggesting that in intestinal adenylate cyclase this site is the primary regulatory site (i.e., primary allosteric effector site) for enzyme activity. These data suggest that sites exist on adenylate cyclase which would allow prostaglandins to serve as the intracellular messengers by which the cell controls its adenylate-cyclase-mediated response to extracellular stimulation, as with hormones.

Regulation of dome formation in differentiated epithelial cell cultures

“…Thus, compounds known to activate adenylate cyclase activity, such as cholera toxin [23] and prostaglandin E , [24] also produced a small but reproducible increase in dome formation. inhibitors of cyclic AMP phosphodiesterase activity [25] , such as theophylline and 1-methyl-3-isobutylxanthine, caused a much more dramatic increase in dome formation.…”

Modification of Myometrial Activity in Vivo by Administration of Cyclic Nucleotides and Theophylline to the Pregnant Rat

“…Bergström, Carlson & Weeks (1968) and Butcher & Baird (1968) have proposed the concept that prostaglandins may feed back to regulate the adenyl cyclase system. There is evidence for such a mechanism in a variety of tissues (Kuehl, Humes, Tarnoff, Cirillo & Ham, 1970;Sato, Szabo, Kowalski & Burke, 1972;Kantor, Tao & Kiefer, 1974). Kantor et al (1974) identified the presence of two independent prosta¬ glandin receptors on, or associated with, the adenyl cyclase.…”

“…There is evidence for such a mechanism in a variety of tissues (Kuehl, Humes, Tarnoff, Cirillo & Ham, 1970;Sato, Szabo, Kowalski & Burke, 1972;Kantor, Tao & Kiefer, 1974). Kantor et al (1974) identified the presence of two independent prosta¬ glandin receptors on, or associated with, the adenyl cyclase. The two receptors appeared to have opposite actions, one regulating the activation of the catalytic site and the other regu¬ lating inhibition.…”

Potentiating effect of bile on enterotoxin-induced diarrhea

“…This difference in propensity for enterotoxin in humans may reflect the absence of factors which participate in the toxin-mediated stimulation of fluid secretion. Prostaglandins have been suggested to play such a role on the basis of their capacity to mimic the effects produced by enterotoxins (6). No correlation, however, between the intestinal levels of these hormones and the propensity for enterotoxic diarrhea has been reported.…”