Chapter 6. Post-Absorption Transport and
Bioavailability
K
ey : To fulfill their biological roles, absorbed minerals must be transported and
assimilated into biological molecules that require them for function. At the organ site, they
must escape the capillary vasculature, penetrate the cell membrane and relocate to specific
compartments within the cell. Macrominerals such as Na+, K+, Cl-, Ca2+, or Mg2+ accomplish
these tasks at concentrations in the millimolar range and are thus able to exploit diffusiondriven mechanisms to overcome membrane barriers. In contrast, microminerals are 1000 times
less concentrated in plasma, which precludes mass-driven access. As we have seen before,
proteins play a prominent role in many transport processes. Those suited for microminerals
can function with the extremely small amounts metal ions present in plasma. Iron, copper,
manganese and zinc transporters deserve special mention in this regard. Here, we key on the
transport and delivery of minerals, metal ions in particular, to outlying cells. Our primary focus
will be on the microminerals. Transport and delivery is linked inextricably with bioavailability as
will be noted. Another key will be on ligands as they present themselves as transporters for
transcelluar and transmembrane movement at the target site and with the target cell.
O
bjectives:
1. To learn the processes of movement minerals from the intestine to peripheral cells,
2. To identify complexes in plasma that transport minerals to their destination,
3. To see how faulty transport could be the basis of mineral deficiencies and diseases
related to deficiencies.
I. OVERALL PERSPECTIVE
Upon leaving the intestine, newly absorbed minerals travel up the portal vein into the
liver. Parenchymel cells in the liver store the minerals, pass them into the bile or release them
into the plasma bound to some carrier protein. Binding to a carrier is necessary for
microminerals, especially micro metal ions, to travel because by themselves they cannot
relocate to outlying (extrahepatic) cells. This failing of the transport system justifies an urgent
need for cell specific carriers that have the power of effective delivery. A case in point is
thyroglobulin, which delivers iodine to the thyroid gland and no other tissue. A list of
transporters is shown in the Table 6.1. Note the prominent role played by albumin. Calcium,
magnesium and some microminerals all requires albumin to travel to the cells. Escaping the
need for protein carriers are the major macronutrient ions such as sodium, phosphate, and
chloride. These ions are free and remain in that state as they move through the plasma to their
cellular destination.
Table 6.1. Minerals Transport Proteins in Serum
Mineral
Calcium
Magnesium
Potassium
Iron (non-heme)
Zinc
Copper
Manganese
Selenium
Iodine
Phosphorus
Chloride
Sodium
Protein
Albumin
Albumin
Albumin
Transferrin
2-macroglobulin, albumin
Ceruloplasmin, albumin
Transferrin, albumin
Selenoprotein P
Thyroglobulin
Lipoproteins
none
none
Comment
50% protein-bound
32% protein-bound
weakly bound
2 iron atoms per protein
weakly attached
firmly bound to both proteins
firmly bound
as selenocyteine or selenomethionine
firmly bound
as phosphate associated with lipids
as free ions
as free ions
Macrominerals, as their name implies, comprise the bulk of the ions in plasma. In
addition to bulk, this category has freedom of form and water-solubility in its favor.
Consequently, macrominerals can form concentration gradients across cell membranes and use
the energy of diffusion to drive inward. Also, macrominerals show fewer propensities to bind
to proteins but instead exist in a quasi equilibrium, which allows minerals that do bind to be
readily set free. Thus, it is fair to say that minerals such as sodium, potassium, chloride exist
mainly as free ions in plasma. In contrast, microminerals are protein bound and never as free
ions. These important points and distinctions are given by a series of rules that apply to all
mineral movement
2. Rules Governing Transport and Delivery of Minerals
Transport proteins not only transport, but they must also deliver, which means they
must have factors built into their structure that allows recognition of the target cell(s). The cell
in turn must have receptors on the membrane for both docking and facilitating the movement
of the mineral into the cell. Thus, the transport protein assures the mineral cargo is placed at
the site where there is high probability of accessing the cell’s interior.
Rule 1: Whereas macrominerals (Ca2+, Mg2+, Na+, Cl- etc.) travel in the blood and access
cells primarily as free ions, the micronutrients (Cu2+, Zn2+, Fe2+, Mn+2) rely on proteins and other
ligands for their movement in the plasma and elsewhere.
Rule2: Targeting microminerals to select organs and locations within cells is a function of
transport proteins in concert with membrane receptors or low molecular weight ligands.
Rule3: Common to both macro-and microminerals are specific portals that lead to
protein channels in the membranes through which minerals course their way into cells from the
exterior, and conversely escapes from the interior of cells. The activity of these portals is
carefully regulated.
Rule 4: In general, because of their bulk, macrominerals use the energy of diffusion and
electrochemical potential across the membrane to gain access to the cytosol from the exterior.
Microminerals, in contrast, exploit energy systems and components derived from the cell’s
metabolism.
These four rules may be summarized by stating that microminerals have a strong dependence
on transport factors, primarily proteins for movement. This is made clear in the second rule.
The third rule puts the case for the microminerals more on a par with macrominerals in
identifying discrete membrane proteins and other components of the delivery mechanisms for
both
Membrane Penetration
When considering membrane passage, there are four mechanistic avenues by which
minerals and other nutrients pass into cells: (1) simple diffusion, (2) facilitated diffusion, (3)
active transport, and (4) receptor-mediated endocytosis. All require energy for the transfer,
but they differ in the source of that energy.
Simple Diffusion
Molecules in a concentrated state are of high energy. The natural tendency is to release
that energy by lowering the concentration. Moving from a higher to a lower concentration is
referred to as diffusion. High concentrations of minerals in the plasma compared to the cytosol
allow minerals to diffuse into the cell through openings in the membrane. Movement will
continue until the concentration of the mineral on either side of the membrane is the same.
This is the point where movement outward and inward occur at the same rate and hence the
two compartments are in equilibrium.
Facilitated Diffusion
When movement through a membrane barrier is aided by another component such as a
carrier in the membrane, the movement is referred to as “facilitated”. Facilitated diffusion
uses the energy of diffusion as the driving force.
Active transport
Active implies another source of energy is needed to affect movement. Many times this
energy is derived with the hydrolysis of ATP concomitant with the movement. Membrane
proteins referred to as ATPases fit this category.
Receptor-mediated endocytosis
A dramatic departure from the other three mechanisms, receptor-mediated is an
internal invagination of the membrane to form a vesicle inside the cell. The segment of the
membrane contains the protein-bound mineral affixed to a receptor protein.
INTRACELLULAR TRANSPORT
The metabolism of minerals within cells is an area still largely unknown. Most of the
advancements have come through studies of individual minerals and extrapolating findings to
other minerals. A mineral that penetrates a cell membrane enters a vast space with many
directions to turn toward and large distances to traverse to reach an internal destination. It
must somehow course its way through the internal milieu to find the organelle or intracellular
compartments that is its ultimate location within the cell. It is generally assumed that the
behavior of minerals in the cytosol of cells closely emulates their movement in plasma. This
means some move as free ions, other require specific proteins with targeting properties to
locate the mineral where it is needed. As befits their status in plasma, monovalent
macrominerals (Na+, K+, Cl-) perform their functions primarily as free-state ions. Divalent
macrominerals in contrast have a greater tendency to form complexes and instead of leaving
target location to chance, use small proteins or organic molecules to aid their transfer. Thus,
calcium ions tend to locate within the endoplasmic reticulum by first binding to calbindin and
magnesium ions tend to form complexes with ATP as their transport agent.
A dramatic departure from these transport mechanisms is seen with microminerals.
Copper, zinc, and iron, for the most part are entrapped in vesicles that move between
organelles including the nucleus and mitochondria. The term “chaperones” describes the
action of small protein that help guide the mineral to the destination or to the site of vesicle
formation. These proteins have structural signals built in that allow entrapment at specific
locations. Chaperones for the microminerals are now coming to light and much of the finding
tend to display an inner world of that may be as complicated as the world in the plasma.
Vesicle transfer is another mode. Well-characterized and potentially dangerous metal
ions such as Fe3+, Zn2+ and Cu+ tend to be sequestered in vesicles that move between
organelles. These vesicles have the advantage of fusing with the inner leaf of the plasma
membrane and spilling their contents outward in the act of effusion. Vesicle that form part of
the trans Golgi network (TGN) are active in mineral transfer for an internal site to an export
site. Both Zn and Cu rely on a family of transfer proteins (recall Zip4 in chapter 5) embedded in
the membrane of vesicles whose main function is to move the ion into the vesicle. A chaperon
(see below) is responsible for bringing the ion to the transfer protein. As a vesicle bound ion,
the movement of the metal is entirely dependent on the movement of the vesicle.
Chaperones for Copper
The best characterized chaperone system is that used for copper movement in cells.
Chaperons for this mineral have a number of functions to perform in assuring the copper
reaches the internal components. CCS is the copper chaperone that delivers copper to the
enzyme superoxide dismutase. Cox17 is a chaperone protein that takes copper to the
mitochondria for eventual incorporation into cytochrome oxidase. Atox1 is a chaperone that
delivers copper to ATP7A, the copper ATPase in the membranes of vesicles that export Cu from
the cell during intestinal absorption. Failure of such proteins gives rise to a copper deficiency in
newborns. The properties of these chaperones will be discussed in greater detail in the
“Copper” chapter.
BIOAVAILABILITY
Bioavailability considerations begin after the nutrient has been passed into the blood. It
basically represents the fraction (or percentage) of an absorbed nutrient that is put to some
functional use. One could even say that bioavailability is at the basis of the first law of nutrition
as quoted by Steven Blezinger which states “no nutrient is absorbed and utilized to the full
extent that it is fed”. By acknowledging that only a fraction of the nutrients taken in the diet is
put to use we are saying how much of what you absorb into your system ultimately becomes
functional. The amount assimilated is expressed as a percentage as indicated in the following
formula definition by O’Dell:
% Bioavailable = % absorbed x % assimilated x 10-2
Based on the definition, digestion is a non-player in bioavailability. Rather, the key factors that
come into play are transport to the cells, passage across the membrane and exchange with
factors in the cell’s milieu. So, if we look for factors that affect bioavailability of minerals, these
are the systems we check first.
2. Key Organs
Digestion
Digestion
Absorption
Absorption
Blood
BloodTransport
Transport
Liver
Liver and
andkidney
kidneyexcretion
excretion
Membrane
Membranetransport
transport
Losses along
the way
Intracellular
Intracellularmovement
movement
Functional
FunctionalSite
Site
Figure 4.1. Overview of a Mineral’s Movement Related to its Bioavailability
3. Body Pools
A nutrient not needed is a nutrient shunned. This simple rule reiterates the common
sense observations that bioavailability is highest when the need is greatest. There will always
be a certain amount of mineral held in reserve. The reserve pool is designed to feed the cell in
the time of need. Generally, this pool is small and tends to be depleted quickly. Body pools are
not static but maintain a dynamic state typical of all biological molecules. This means they
maintain a steady-state of synthesis and degradation of their components. In the case of iron
this could mean adding iron to ferritin and breaking down the ferritin to release the iron.
Although on can interpret this as a waste, it fits the needs of the cell in being able to shift
between synthesis and degradation favoring one or the other depending on circumstances.
SUMMARY
Proteins play an important role in the nutrition and metabolism of minerals. Specific
proteins act as large ligands that bind minerals for transport in the blood, and enter cells by
movement across membranes. In essence, they permit minerals to transcend barriers and to
locate targets within cells. As a complex, proteins heighten the metal’s solubility and
membrane penetrating properties at levels where diffusion-driven events are unworkable.
Some are receptors that are part of the cell’s uptake system. This puts sensitivity on a par with
the protein’s binding affinity for the metal. The latter statement heeds the notion that ligands
can be forceful determinants of the selectivity and sensitivity of the transport process in
general. By binding to their surface, the fate of the metal depends on what happens to the
protein. If the protein enters the cell, the metal ion goes with it.
There are both similarities and differences in movement mechanics between the two
classes of minerals. Similarities can be found in the requirement for specific passage way
components that have the power of discriminating among the different minerals. Differences
lie in the source of energy and the need for a piggy-back factor to carry the mineral through the
lipid bilayer. Each mineral seems to have its own set of principles that must be followed. The
best way to fully appreciate the mechanistic factors that come into play is to treat each mineral
element as a separate system and consider one element borrowing another’s transport system
as a basis for mineral-mineral interference.
PROBLEMS
1. From Table 6.1, name at least 5 proteins in the plasma that take part in post-absorption
transport of minerals to cells. Of those named, which ones are specific for just one mineral?
2. Of the 4 different mechanisms for penetrating a cell membrane, which one seems more a
propos to iron uptake? For calcium uptake?
3. Phosphate is perhaps the most common mineral complex in cells. How is phosphate
transported into cells from the plasma. How does phosphate, e.g., HPO4= exist the cytosol of
cells. Describe a reaction in which HPO4= is a substrate.
4. Describe the status of potassium ions in the cell. Is it a free ion or a complex? How does
potassium ion get into a cell? Predict the relative ratio of potassium outside/inside?
5. Of the chaperones for microminerals, those for copper are the best understood. At least 4
have been identified. Complete the chart below.
Required to activate an antioxidant enzyme
Cellular respiration depends on it
Transfers copper across the intestine during
absorption
Required to incorporate copper into
ceruloplasmin
6. Suppose a system took in 10 mg of iron of which half was absorbed and 1.2 mg was rendered
functional. Calculate the bioavailability of iron in this system?