Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 22 Trace Minerals
they are intimately associated with the functions of specific
organic molecules, mostly proteins with enzymatic
properties. When metals function to facilitate enzymatic
catalysis, they typically fall into two categories, metalloenzymes
and metal-enzyme complexes. Stability constants
that define metal binding dictate whether metalloenzyme
or metal-enzyme complex is the best designation (Reedijk
and Bouwman, 1999; Taylor, 2002 ). For metalloenzymes
there is often good stoichiometry between the moles of
metal bound per mole of protein or protein subunit following
purification. Metalloenzymes have metal-binding
constants of 10 8 to 10 9 or greater. Metal protein complexes
have constants of 10 5 or less. Metals in such complexes are
easily dissociated upon dialysis of the complex ( Reedijk
and Bouwman, 1999 ).
TABLE 22-1 Trace Elements Essential for the
Development and Health of Mammals and Birds
Subject to Natural
Deficiencies
Experimentally
Produced a
Cobalt
Arsenic
Copper
Chromium
Iodine
Fluoride
Iron
Nickel
Manganese
Silicon
Molybdenum
Vanadium
Selenium
Lithium
Zinc
Boron
a
Defi ciency induced experimentally using purifi ed diets in a rigidly controlled
environment; “ defi ciency ” is often dependent on and a function of the controlled
environment.
Trace elements that are nutritionally essential are localized
to the fourth and fifth rows of the periodic table. All
have incompletely filled d orbitals, except for Cu and Zn.
How a given metal facilitates catalytic functions is related
in part to its ability to engage in redox (the loss or gain of
an electron[s]) or modulate an energy excitable transition
state during a catalytic event. Such modulations in protein
transition states are sometimes referred as entasis. The entasis
(structural dictating) domains of most proteins utilize O,
S, or N as electron donors (Riordan and Valle, 1974). When
associated with given complexes, if one or two outer shell
valence electrons are involved, the result is one oxidation
state (e.g., Na 1 , Ca 2 , or Mg 2 ); two or more valence electrons
can result in two oxidation states (Fe 2 or Fe 3 ). The
use of electrons in orbitals other than the outermost valence
orbitals (e.g., transition metals), however, can have a variety
of oxidation states. Some metals can also function as a
Lewis acid (i.e., can accept electrons from a base). Enzymes
that utilize this property act as acid catalysis (e.g., Zn and
the hydrolysis of phosphate esters by alkaline phosphatase
(Taylor, 2002 ).
B . Typical Configurations of Metal
Complexes
In simple metal complexes, the basicity of the electron
donating group and the ability to approach the metal ion
(steric effects) are the primary factors that influence stability.
However, when the electron donor groups are bound
together into a single molecule capable of binding a given
metal, the importance of steric effects is greatly increased
and stability depends on a number of other factors including
the size and number of rings formed. Five- or six-membered
rings have more stability; usually five-membered rings are
more stable than six-membered rings. Four-membered rings
are rare. Rings larger than six members are less unstable.
TABLE 22-2 Cobalt, Copper, Manganese, Molybdenum, Selenium, and Zinc Requirements for Young and Adult
Dogs, Swine, Sheep, and Beef and Dairy Cattle Expressed as Mg per Kg of Ration for Adequate Intakes a
Dogs Swine Sheep Beef Cattle Dairy Cattle
Co 30–60 mg as vitamin B 12 30–60 mg as vitamin B 12 0.1–0.3 mg as Co 0.1–0.3 mg as Co 0.1–0.3 mg as Co
Cu 5–10 5–10 5–10 5–10 5–10
Mn 3–20 3–20 20–40 20–40 20–40
Mo b 1–2 1–2 1–2 1–2 1–2
Se 0.25–0.3 0.25–0.3 0.15–0.40 0.15–0.4 0.15–0.4
Zn 80–100 80–100 30–60 30–60 30–60
a
Values were taken from several industrial and NRC sources (Committee on Animal Nutrition, NRC, 1985; Subcommittee on Dairy Cattle Nutrition, NRC, 2001; Subcommittee on
Laboratory Animal Nutrition, NRC, 1995; Subcommittee on Mineral Toxicity in Animals, NRC, 1980; Subcommittee on Swine Nutrition, NRC, 1998; The Salt Institute,
www.saltinstitute.org/index.html).
b
For ruminant animals, the Cu:Mo ratio should exceed 5 or more given the interactions and negative effects Mo has on Cu availability (see text).