C35 its synthesis is induced by vitamin d and it is

Info icon This preview shows pages 144–147. Sign up to view the full content.

View Full Document Right Arrow Icon
kDa) formerly known as ICaBP (see Section V.C).35 Its synthesis is induced by vitamin D, and it is mainly found in mammalian intestines. The porcine and bovine calbindin D 9K has a Ca 2 + binding constant of K B = 3 X 10 8 M - 1 in low ionic strength media 37 and K B = 2 X 10 6 M -1 in the presence of 1 mM Mg 2 + and 150 mM K + .38 The concentration of calbindin D 9K in epithelial cells can reach millimolar levels, 35 which could allow it to facilitate Ca 2 + diffusion across the cytosol. This was first suggested by Williams, subsequently elabo- rated by Kretsinger et aI. in 1982,39 and later demonstrated in a model cell by Feher. 40 The basic idea is that, although the diffusion rate of Ca 2 + ions (~1O ~5 cm 2 s -1) is higher than for the (Ca 2+)2 calbindin complex
Image of page 144

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
123 S ~ ~ Ca 2 + c Ca 2 + OJ E .2 ·0 vesicles? 0 7 Na+ I CaBP? ~ mitochondria brush cytosolic compartment basal- border lateral membranes Figure 3.8 A scheme representing some of the known and hypothetical molecular participants in the trans- port of Ca 2 + across intestinal epithelial cells. Transport across the brush-border membrane is generally assumed to be passive or to be facilitated by a carrier (1M Cal), and is also influenced by vitamin D. Transport through the cell may be in vesicles and/or in association with Ca 2+_ binding proteins (CaBP), notably calbindins D 9k (mammals) or D 28k (avians). Temporary storage or buffering of Ca 2 + may be through cytosolic CaBPs, mitochondria, endoplasmic reticula (ER), or other organelles. Transport of Ca 2 + out of the cell through the basal-lateral membranes is energetically uphill, and appears primarily accomplished by a Ca 2 + -ATPase and possibly to some extent by a Na 2 + _Ca 2 + antiport. Adapted from Reference 35. ( ~ 0.2 X 10 - 5 cm 2 s - I), the fact that the concentration of the latter complex may be about 10 3 times higher than that of free Ca 2+ will result in an increased net calcium transport rate. Calbindin would, in fact, act very much like myo- globin in facilitating oxygen transport through muscle tissue. Plausible as the above mechanism may seem, it may, however, not be the whole truth. An alternative mechanism is vesicular transport. In chicken intes- tine it has been shown that the only epithelial organelles that increased in Ca 2+ content as a result of calcitriol treatment were the lysosomes. 41 The result lends support to a transport mechanism involving Ca 2+ uptake across the brush-bor- der membrane by endocytic vesicles, fusion of these vesicles with lysosomes, and possibly also delivery of Ca 2+ to the basal lateral membrane of the epithe- lial cell by exocytosis. This process would also explain the vitamin-D-induced alterations in brush-border-membrane lipid compositions as a consequences of preferential incorporation of certain types of lipids into the vesicles. Interest- ingly, the lysosomes in the chicken studies also contained high levels of calbin-
Image of page 145
124 3 I CALCIUM IN BIOLOGICAL SYSTEMS din D 28k -a type of vitamin-D-induced Ca 2+ -binding protein found in avian intestines-making it conceivable that this protein acts as a "receptor" for Ca 2 +
Image of page 146

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 147
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern