Biology Reference
In-Depth Information
acyl chains located at positions
-2, respectively. Both products of the hydrolysis,
free fatty acids and lyso-phospholipids, are harmful to membranes and so are normally
present in trace amounts. Significant quantities of a free fatty acid produce a detergent effect
that can result in membrane phospholipids dissolving into fatty acid micelles (
Figure 5.1
).
The remaining portion of the phospholipid is referred to as a lyso-phospholipid. Lyso refers
to the molecule's ability to disrupt or lyse a membrane. Lyso-phospholipids are cone shaped
with a wide polar head and a narrow apolar tail. Molecules with this shape do not readily fit
into regular lamellar phase packing, thus distorting the membrane (see Chapter 10). High
levels of free fatty acids and lyso-phospholipids are usually the result of artifacts arising
from the membrane isolation procedure or arise from membranes isolated from an aged or
diseased cell. An exception to this is found in chromaffin granule membranes that appear
to be naturally high in free fatty acids. Chromaffin granules are organelles in chromaffin cells
of the adrenal medulla, where epinephrine and norepinephrine are synthesized, stored, and
released.
The action of phospholipase C on phospholipids produces diacylglycerol (DAG) and the
water-soluble phospho-alcohol characteristic of the phospholipid type. As discussed above,
DAG is a potent activator of several membrane proteins.
sn
-1 and
sn
Lipid-Soluble Vitamins
A vitamin
[39]
is an organic compound that is essential in the diet in small amounts
because it cannot be synthesized in sufficient quantities by an organism. Vitamins are classi-
fied as either being water-soluble or lipid-soluble. In humans there are 4 lipid-soluble (A, D,
E, and K) and 9 water-soluble (8 B vitamins and vitamin C) vitamins. Here we are interested
in the major lipid-soluble vitamins A, D, and E (
Figure 5.26
) due to their favorable partition-
ing into membranes.
The family of vitamin As
[40]
exist in three oxidation states; alcohol (retinol), aldehyde
(retinal) and acid (retinoic acid) (
Figure 5.26
). Vitamin As are involved in complex roles as
mediators of cell signaling and regulators of cell and tissue growth and differentiation.
The first recognized vitamin A, retinol, was discovered around 1913 by Elmer McCollum
and Marguerite Davis in butterfat and cod liver oil. The vitamin was shown to prevent
and reverse night blindness. Retinol is now added to skin creams where it is advertised
as preventing acne. It also functions as a membrane antioxidant. In the eye, retinal is
known to attach to the protein opsin via a Schiff base linkage to lysine, creating rhodopsin
or visual purple. Retinal is therefore essential for color vision. Gradients of retinoic acid
control the activation of many developmentally important genes.
The structure of Vitamin D (
Figure 5.26
) indicates it is a metabolic product of choles-
terol that results from the opening of ring B in the sterol nucleus. Although vitamin D
3
itself serves no biological function, it is converted to 1,25-dihydroxycholecalciferol,
a hormone that regulates calcium uptake in the intestine and calcium levels in the kidney
and bone. Vitamin D deficiency results in bone defects and the disease rickets. For this
reason it has often been said that vitamin D has 'hormonal' activity. Vitamin D was
discovered by Edward Mellanby in 1921 and was shown to have anti-rickets activity.
Vitamin E (
Figure 5.26
) is a family of related compounds composed of eight different
forms, the most biologically important being
a
-tocopherol
[41]
. The term
a
-tocopherol is
