Page 351 - Color Atlas of Biochemistry

342 Tissues and organs

Calcium metabolism tion in estrogen levels. Estrogens normally
inhibit the stimulation of osteoclast differen-
tiation by osteoblasts. If the effects of estrogen
A. Functions of calcium
decline, the osteoclasts predominate and ex-
2+
The human body contains 1–1.5 kg Ca ,most cess bone removal occurs.
of which (about 98%) is located in the mineral The effects of the steroid hormone calcitriol
substance of bone(seep. 362). (see p. 330)in boneare complex. On theone
In addition to its role as a bone component, hand, it promotes bone formation by stimu-
calcium functions as a signaling substance. lating osteoblast differentiation (top). This is
Ca 2+ ions act as second messengers in signal particularly important in small children, in
transduction pathways (see p. 386), they whom calcitriol deficiency can lead to miner-
trigger exocytosis (see p. 228) and muscle alization disturbances (rickets; see p. 364). On
contraction (see p. 334), and they are indis- the other hand, calcitriol increases blood Ca 2+
pensable as cofactors in blood coagulation (see levels through increased Ca 2+ mobilization
p. 290). Many enzymes also require Ca 2+ for from bone. An overdose of vitamin D (chole-
their activity. The intracellular and extracel- calciferol), the precursor of calcitriol, can
lular concentrations of Ca 2+ are strictly regu- therefore have unfavorable effects on the
lated in order to make these functions possi- skeleton similar to those of vitamin deficiency
ble (see B, C, and p. 388). (hypervitaminosis; see p. 364).
Proteins bind Ca 2+ via oxygen ligands, par-
ticularly carboxylate groups and carbonyl
groups of peptide bonds. This also applies to C. Calcium homeostasis
the structure illustrated here, in which a Ca 2+ Ca 2+ metabolism is balanced in healthy adults.
ionis coordinated by the oxygenatoms of Approximately 1g Ca 2+ is taken up per day,
carboxylate and acid amide groups. about 300 mg of which is resorbed. The same
amount is also excreted again. The amounts of
Ca 2+ released from bone and deposited in it
B. Bone remodeling
per day are much smaller. Milk and milk prod-
Deposition of Ca 2+ in bone (mineralization) ucts, especially cheese, are particularly rich in
and Ca 2+ mobilization from bone are regu- calcium.
lated by at least 15 hormones and hormone- Calcitriol and parathyroid hormone, on the
like signaling substances. These mainly influ- one hand, and calcitonin on the other, ensure
ence the maturation and activity of bone cells. a more or less constant level of Ca 2+ in the
Osteoblasts (top) deposit collagen, as well blood plasma and in the extracellular space
as Ca 2+ and phosphate, and thereby create (80–110 mg 2.0–2.6 mM). The peptide para-
new bone matter, while osteoclasts (bottom) thyroid hormone (PTH; 84 AA) and the steroid
+
secrete H ions and collagenases that locally calcitriol (see p. 374) promote direct or indi-
dissolve bone (bone remodeling). Osteoblasts rect processes that raise the Ca 2+ level in
and osteoclasts mutually activate each other blood. Calcitriol increases Ca 2+ resorption in
by releasing cytokines (see p. 392) and the intestines and kidneys by inducing trans-
growth factors. This helps keep bone forma- porters. Parathyroid hormone supports these
tion and bone breakdown in balance. processes by stimulating calcitriol biosynthe-
2+
The Ca -selective hormones calcitriol, par- sisin the kidneys(see p. 330). In addition, it
athyroid hormone, and calcitonin influence directly promotes resorption of Ca 2+ in the
this interaction in the bone cells. Parathyroid kidneys (see p. 328) and Ca 2+ release from
hormone promotes Ca 2+ release by promoting bone (see B). The PTH antagonist calcitonin
the release of cytokines by osteoblasts. In (32 AA) counteracts these processes.
turn, the cytokines stimulate the develop-
ment of mature osteoclasts from precursor
cells (bottom). Calcitonin inhibits this process.
At thesametime, it promotes thedevelop-
ment of osteoblasts (top). Osteoporosis, which
mainly occurs in women following the men-
opause, is based (at least in part) on a reduc-

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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