Page 86 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN008C-380 June 29, 2001 16:42

Lipoprotein/Cholesterol Metabolism 653

TABLE III Properties of the Apolipoproteins
Molecular Concentration
Apolipoprotein weight (Da) (mg/dl) Tissue origin Function
Apo-A1 28,300 90–130 Intestine, liver LCAT activator
Apo-A2 17,000 30–50 Intestine, liver Unknown
Apo-A4 46,000 10–30 Intestine, liver Unknown
Apo-B100 516,000 80–100 Liver Cholesterol, triglyceride transport,
receptor recognition
Apo-B48 264,000 — Intestine Triglyceride, cholesterol transport
Apo-C1 6,500 4–7 Liver LCAT activator
Apo-C2 8,800 3–8 Liver LPL activator
Apo-C3 8,750 8–15 Liver LPL inhibitor
Apo-D 33,000 10 Liver, intestine, pancreas, Unknown
kidney, adrenals, brain
Apo-E 35,000 3–6 Liver, macrophages, brain, Receptor recognition
Adrenal
CETP 74,000 — Spleen, liver, small Cholesterol ester transfer
intestine, adrenal
Lp(a) 400–700 0.4–80 Liver, testes, brain Unknown

X. OTHER LIPOPROTEIN RECEPTORS B. Cubulin
Cubulin is a 460-kDa protein localized to epithelial cells,
A. LRP/α 2 -Macroglobulin Receptor
such as those of the kidney. It plays a role in albumin
The lipoprotein receptor-related protein (LRP)/α 2 - reabsorption; its absence leads to loss of albumin in the
macroglobulin receptor (LRP) has a mass about five times urine. Cubulin also mediates the uptake of the major HDL
larger than the LDL receptor. It contains many of the protein apo-A1 by the kidney.
same structural domains as the LDL receptor, including a
cysteine-rich repeat domain that binds to lipoproteins. In
C. Megalin (gp330)
vitro studies have shown that the LRP does not bind to apo-
B100, but binds to VLDL particles that have been enriched This receptor is a member of the LDL receptor superfam-
in apo-E. The binding of LRP to its ligands is dependent ily. It is the antigen that elicits an autoimmune response
upon cell surface proteoglycans. These are proteins con- leading to a disease called Heymann’s glomerulonephritis.
taining sulfated carbohydrate residues. It interacts with cubulin and is thought to function together
In addition to binding lipoproteins, the LRP also binds with cubulin in the uptake of apo-A1 by the kidney.
to α 2 -macroglobulin, a plasma protein that binds to pro-
teases and, upon binding, is cleared by the liver. The LRP
also binds to plasminogen activators and their inhibitors, D. A VLDL Receptor (?)
such as tissue-type plasminogen activator, urokinase-
A cDNA encoding another member of the LDL receptor
type plasminogen activator, and their corresponding
family is expressed in muscle and adipose tissue, but not in
inhibitors.
liver. It binds to apo-E-containing lipoproteins in vitro,but
not to LDL. Based on its tissue localization and binding
properties, this protein has been postulated to be a receptor
TABLE IV Apolipoproteins of the Plasma for triglyceride-rich lipoproteins.
Lipoproteins
Chylomicrons A-1, A-2, A-4, B-48, C-1, C-2, C-3, E
E. CD-36
VLDL B-100, C-1, C-2, C-3, E, trace amounts
of A-1, A-2, and A-4
CD-36 is expressed in a variety of tissues, including
LDL 90–98% apo B-100
adipocytes and macrophages. It functions in fatty acid
HDL A-1, A-2, sometimes, E
transport and, like scavenger receptors, binds a variety