title: Characterization of the vitamin D-dependent Ca2+-binding sites in rat intestinal Golgi-enriched membrane fractions.
abstract: Rat intestinal Golgi-enriched membrane fractions take up Ca2+ by a vitamin D-dependent process that has been shown to recover within 15 min of repletion of vitamin D-deficient animals with intravenous 1,25-dihydroxycholecalciferol. The present paper reports studies characterizing the Ca2+-binding sites of these membrane fractions. Equilibrium binding of Ca2+ at concentrations between 5 and 400 microM showed significant decreases at all concentrations in membranes derived from vitamin D-deficient animals when compared with normal control-diet-fed animals. The predominant class of binding sites had a relatively high affinity for Ca2+ (KD approx. 3 microM). Vitamin D-deficiency did not change the affinity of this class of site, but decreased the number from 347 +/- 26 to 168 +/- 50 nmol of Ca2+ bound/mg of protein (means +/- S.D.). Mg2+ inhibited binding only at low Ca2+ concentrations, and the characteristics of this binding suggested positive co-operativity between two binding sites. Equimolar concentrations of Zn2+, La3+, Pb2+ and Mn2+ inhibited Ca2+ binding by over 50%. Increased ionic strength decreased Ca2+ binding by no more than half. Binding was maximal at pH 7.5 and half-maximal at pH 6.3. The large number of binding sites with relatively high affinity for Ca2+ suggests that it is unlikely that this binding is to any specific protein or to non-specific sites present on many proteins, and that the most likely sites are lipid molecules.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002412:Cations / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D006056:Golgi Apparatus / Q000187:drug effects / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D014807:Vitamin D / Q000494:pharmacology*
chemical_list: D002412:Cations; D014807:Vitamin D; D002118:Calcium