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CASE FILES: BIOCHEMISTRY
Niacin is a vitamin that is used in high doses to treat hypercholesterolemia.
Niacin acts to decrease VLDL and LDL plasma levels. Its mechanism of action
is not clearly understood but probably involves inhibition of VLDL secretion,
which in turn decreases the production of LDL. Niacin inhibits the release of
free fatty acids from adipose tissue which leads to a decrease of free fatty acids
entering the liver and decreased VLDL synthesis in the liver. This decreases the
availability of VLDL for conversion to LDL (containing cholesterol esters).
Niacin also increases high-density lipoprotein (HDL) (the “good cholesterol”)
by an unknown mechanism.
C O M P R E H E N SIO N Q U E ST IO N S
[34.1] Which of the following compounds directly inhibits the expression of
the HMG-CoA reductase gene?
A. Squalene
B. HMG-CoA
C. Lanosterol
D. Isopentenyl pyrophosphate
E. Cholesterol
[34.2] You decide to treat a patient who has very high levels of serum cho-
lesterol with the statin drug Lipitor (atorvastatin). You know that this
drug acts in the metabolic pathway leading to the synthesis of choles-
terol. The substrate for the enzyme inhibited by the statin drugs is
which of the following?
A. Acetoacetyl-CoA
B. HMG-CoA
C. Farnesol pyrophosphate
D. Isopentenyl pyrophosphate
E. Mevalonate
[34.3] Which of the following vitamins can be used in high doses to treat
hypercholesterolemia?
A. Niacin
B. Riboflavin
C. Pyridoxine
D. Folic acid
E. Thiamine
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