Introduction to Human Nutrition

234 Introduction to Human Nutrition

Table 9.22

Excretion, retention, and possible biological roles of the ultratrace elements

Element

Organs of high content (typical concentration)

Major excretory route after ingestion

Molecules of biological importance

Possible biological role

Aluminum

Bone (1–12

μg/g)

Lung (35

μg/g)

Urine; also signifi cant

amounts in bile

Aluminum binds to proteins, nucleotides and

phospholipids; aluminum-bound transferrin apparently is a transport molecule

Enzyme activator

Arsenic

Hair (0.65

μg/g)

Nails (0.35

μg/g)

Skin (0.10

μg/g)

Urine

Methylation of inorganic oxyarsenic anions occurs in

organisms ranging from microbial to mammalian; methylated end-products include arsenocholine, arsenobetaine, dimethylarsinic acid and methylarsonic acid; arsenite methyltransferase and monomethylarsonic acid methyltransferase use S-adenosylmethionine for the methyl donor

Metabolism of methionine, or involved in

labile methyl metabolism; regulation of gene expression

Boron

Bone (1.6

μg/g)

Fingernails (15

μg/g)

Hair (1

μg/g)

Teeth (5

μg/g)

Urine

Boron biochemistry essentially that of boric acid, which

forms ester complexes with hydroxyl groups, preferably those adjacent and

cis

, in organic

compounds; fi ve naturally occurring boron esters (all antibiotics) synthesized by various bacteria have been characterized

Cell membrane function or stability such that it

infl uences the response to hormone action, transmembrane signaling or transmembrane movement of regulatory cations or anions

Bromine

Hair (30

μg/g)

Liver (40

μg/g)

Lung (6.0

μg/g)

Testis (5.0

μg/g)

Urine

Exists as Br

− ion

in vivo

, binds to proteins and amino

acids

Electrolyte balance

Cadmium

Kidney (14

μg/g)

Liver (4

μg/g)

Urine and gastrointestinal

tract

Metallothionein, a high sulfhydryl-containing protein

involved in regulating cadmium distribution

Involved in metallathionein metabolism and

utilization

Germanium

Bone (9

μg/g)

Liver (0.3

μg/g)

Pancreas (0.2

μg/g)

Testis (0.5

μg/g)

Urine

None identifi ed

Role in immune function

Lead

Aorta (1–2

μg/g)

Bone (25

μg/g)

Kidney (1–2

μg/g)

Liver (1–2

μg/g)

Urine; also signifi cant

amounts in bile

Plasma lead mostly bound to albumin; blood lead binds

mostly to hemoglobin but some binds a low molecular weight protein in erythrocytes

Facilitates iron absorption and/or utilization

Lithium

Adrenal gland (60 ng/g)Bone (100 ng/g)Lymph nodes (200 ng/g)Pituitary gland (135 ng/g)

Urine

None identifi ed

Regulation of some endocrine function