A specific binding moiety for 1,25-dihydroxyvitamin D3 in basal lateral membranes of carp enterocytes

Abstract

Carp (Cyprinus carpio), a freshwater fish that lives in a low-calcium environment, and Atlantic cod (Gadus morhua), a seawater fish that lives in a high-calcium environment, were studied for the presence of a novel membrane binding protein (“receptor”) for the vitamin D metabolite, 1,25-dihydroxyvitamin D3[1,25(OH)2D3]. Basal lateral membranes from enterocytes of either species were prepared and analyzed for specific saturable binding. Membranes from carp revealed a dissociation constant of 1.23 nM with a maximal binding capacity of 212 fmol/mg protein. In comparison, membranes from Atlantic cod enterocytes revealed very low and nonsignificant levels of specific binding. The [3H]1,25(OH)2D3 binding activity in carp was further characterized for protein dependence, detergent extractability, and competition for binding with the metabolites 25(OH)D3 and 24 R,25(OH)2D3. Finally, introduction of 1,25(OH)2D3 to isolated carp enterocytes enhanced protein kinase C activity within 5 min, whereas intracellular Ca2+ concentrations were unaffected by a range of 1,25(OH)2D3 concentrations, as judged by fura 2 fluorescence. Thus the binding moiety may be a putative plasma membrane receptor for vitamin D, because it is functionally coupled to at least one signal transduction pathway.