Uptake and binding of prostaglandins in a marine bivalve, Modiolus demissus

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Abstract
The existence of specific PGA2 binding sites was demonstrated in the 12,000 × g supernatant fraction of homogenized gill tissue from Modiolus demissus (Geukensia demissus). The binding of prostaglandins at these sites was pH‐dependent, time‐dependent, and reversible. The binding was more specific for prostaglandin A2 (PGA2) than prostaglandin E2 (PGE2) or prostaglandin F2α (PGF2α). Binding was enhanced by calcium and magnesium. For calcium the binding kinetics exhibited a Km of 7 mM; for magnesium the Km was 15 mM. The amount of PGA2 bound/mg protein at saturating cation concentration was determined to be 0.20 pmoles/mg for both calcium and magnesium. A dissociation constant (Kd) of 9.8 × 10−5 M was calculated by Scatchard plot analysis for the PGA2 binding site. Binding was inhibited by 20 mM Cu+ +, 10−3 M ATP and 10−3 M GTP. These low‐affinity binding sites may perform an important role in the physiological action of prostaglandins in marine bivalves.

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