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chapter37
Mineral Metabolism
TA B LE 37-6
Some Mammalian Zinc Proteins
Enzyme
Source
Molecular
Weight
Zn Atoms/
Protein
Molecule
Functions/Comments
Alcohol dehydrogenase
Human liver
87,000
4
Requires 2NAD+; two zinc atoms
used for catalysis; function
of other two not known but may
stabilize enzyme.
Alkaline phosphatase
Human placenta
125,000
~3
Unknown function; role of Zn2+
unknown but needed for activity.
Leucine aminopeptidase
Porcine kidney
300,000
6
N-terminal exopeptidase; Zn binds
at two sites, one for catalysis,
the other for regulation.
5'-AMP aminohydrolase
Rat muscle
290,000
2
—
Carbonic anhydrase B
Human erythrocyte
26,600
1
Catalyzes reversible hydration
of C 02; needed for C 0
2
transport,
buffering of ECF, gastric and renal
acid secretion; Zn is catalytic.
Carboxypeptidase A
Bovine pancreas
34,500
1
C-terminal exopeptidase;
involved in digestion of proteins;
Zn functions in catalysis as a
Lewis acid.
Glutamate dehydrogenase
Bovine liver
1
,
0 0 0 , 0 0 0
2 -4
—
a 2-Macroglobulin
Human serum
840,000
3-8
—
Malate dehydrogenase
Bovine heart
40,000
1
—
Metallothionein
Human renal cortex
10,500
3
Usually also contains 4-5 Cd;
may serve as source of Zn.
Rhodanese (sulfur
transferase)
Bovine liver
37,000
2
Metabolizes SCN- .
The zinc content of human blood is
8 . 8
/xg/mL, of which
80-90% is within erythrocytes, mostly in carbonic an-
hydrase. Zinc binds to hemoglobin, increasing its oxy-
gen affinity. By binding to erythrocyte membranes, it in-
creases the flexibility of irreversibly sickled cells
in vitro
but plasma levels high enough to be therapeutic cannot be
achieved. Leukocytes and platelets also contain an appre-
ciable amount of zinc.
Plasma zinc levels vary with sex, age, time of day, ge-
ographic location, and time elapsed since the last meal
prior to phlebotomy. The normal range for an adult is
probably 70-95 /xg/dL of plasma. Approximately 60%
of zinc in plasma is bound to albumin, 30-40% to an «
2
-
macroglobulin of unknown function, and a small amount
to transferrin (Chapter 29) and amino acids, particularly
cysteine and histidine. Copper does not compete with zinc
for binding sites on albumin. Zinc newly absorbed from
the intestine is albumin-bound. This zinc is the fraction
that changes most in disease states and that correlates best
with total serum zinc. The zinc bound to amino acids ac-
counts for urinary excretion of zinc, and it may be utilized
by some tissues.
The RDA for zinc depends on age. For infants younger
than
6
months, it is 3 mg; from
6
months to 1 year, 5 mg; for
ages 1-10 years, 10 mg; and more than 10 years, 15 mg.
For women, it increases during pregnancy (20 mg) and
lactation (25 mg). A typical daily diet in the United States
supplies 12-15 mg (about 200 /xmol), of which 3CM-0%
is absorbed. Dietary fiber binds and decreases the absorp-
tion of zinc. Human zinc deficiency was first reported in
boys from Iran and Egypt who were small in stature, hy-
pogonadal, and mildly anemic. The zinc content of their
hair, plasma, and erythrocytes was below normal. Etio-
logical factors included high dietary phytate content, loss
