| Abstract: | A high affinity binding site for leukotriene C4 (LTC4), one component of slow reacting substance of anaphylaxis, has been identified in a membrane preparation from rat lung. As measured by a filtration technique, 3H]LTC4 binding was saturable, specific, reversible, and heat-sensitive. In the presence of 20 mM CaCl2, the dissociation constant (KD) was 41 +/- 9 nM and the maximum number of binding sites (Bmax) was 31 +/- 10 pmol/mg of protein. Specificity was demonstrated by competition studies in which LTC4 had a Ki of 40 nM against specifically bound 3H]LTC4, whereas leukotriene D4 (LTD4) had a Ki of 4 microM. The stereoisomers (5R, 6R) LTC4, (5S, 6S) LTC4, and (5R, 6S) LTC4 had Ki values 3-, 15-, and 25-fold higher than that of natural (5S, 6R) LTC4. Leukotrienes E4 and B4, several prostaglandins and fatty acids, glutathione, and platelet activating factor were even less effective with Ki values above 10 microM. A slow reacting substance of anaphylaxis antagonist, FPL 55712, which, in some systems, distinguishes LTC4- from LTD4-induced contractions, was a weak competitor with a Ki of 16 microM. Serine-borate complex which inhibits gamma-glutamyl transpeptidase, an enzyme responsible for LTC4 metabolism, did not alter binding. In addition, 100 microM FPL 55712 did not reduce metabolism. These observations suggest that the binding observed for LTC4 may represent association with a physiological receptor for this molecule which has a relatively low affinity for LTD4. |
