Question

I need help with answering these case study questions. its for cell biology class.

Background: Researchers were looking into the cause of a disease called Familial Hypercholesterolemia (FH) which is a genetic disorder that results in severely high serum cholesterol leading to blocked arteries and fatal heart attacks in diseases afflicted individuals. individuals Control fibroblast from normal individuals and fibroblast derived from FH individuals were grown in cell culture and tested as follows: Figure 1. The question of whether cholesterol synthesis or uptake was addressed. In this study, normal fibroblasts were tested for HMG reductase activity (Y axis) which is the rate limiting enzyme in the cholesterol synthesis pathway. Lipoprotein fractions are shown as open squares), unfractionated calf serum (closed circles) or non-lipoprotein fractions of calf serum (open triangles) 30 20 30 10 20 SERUM FRACTION (vol % ) nor From this study (Figure 1) answer the following questions: Why do you suppose different fractions of serum were used in this study? 1. Why was HMG reductase activity measured? 2. 3. What was the effect of the different fractions on HMG reductase level? Based on your answer to 2, what can be concluded from Figure 1 4. (uajoud jo Buu Jed ujuoud) AIALLOY

Answer 1

1. Familial hypercholesterolemia (FH) is characterized clinically by (a) an elevated concentration of LDL, the major cholesterol-transport lipoprotein in human plasma. Inheritance as an autosomal dominant trait with a gene dosage effect—that is, homozygotes are more severely affected than are heterozygotes. Homozygotes number 1 in 1 million persons. They have severe hypercholesterolemia (650 to 1000 mg/dl).

Cholesterol: classified on the basis of density

-least dense particles known as chylomicrons are normally found in the blood only after fat-containing foods have been eaten
-Most labs measure total cholesterol, total triglycerides and cholesterol found in the HDL fraction which is EASILY precipitated from serum

HDL levels are

higher in women
-also many with apparently high total cholesterol levels have favorable lipid profiles

When LDL rise

-as age increases
-acutre illness
Exception: Serum triglyceride levels in patients with pancreatitis

2. Since the reaction catalyzed by HMG-CoA reductase is the rate-limiting step in cholesterol synthesis, this enzyme represents the sole major drug target for contemporary cholesterol-lowering drugs in humans. Normally in mammalian cells, this enzyme is suppressed by cholesterol derived from the internalization and degradation of low-density lipoprotein (LDL) via the LDL receptor as well as oxidized species of cholesterol. Competitive inhibitors of the reductase induce the expression of LDL receptors in the liver, which in turn increases the catabolism of plasma LDL and lowers the plasma concentration of cholesterol, which is considered, by those who accept the standard lipid hypothesis, an important determinant of atherosclerosis. This enzyme is thus the target of the widely available cholesterol-lowering drugs known collectively as the statins.

The rate-determining step in the biosynthesis of cholesterol is the synthesis of mevalonate catalyzed by hydroxymethylglutaryl-CoA reductase. This enzyme is subject to feedback regulation by the negative modulator cholesterol.

3.  HMG-CoA reductase (HMGR), a highly conserved, membrane-bound enzyme, catalyzes a rate-limiting step in sterol and isoprenoid biosynthesis and is the primary target of hypocholesterolemic drug therapy.