CLINICAL CHEMISTRY CHAPTER 12 LIPIDS AND LIPOPROTEINS

INTRODUCTION When was the last time you ordered a Double Bacon Cheeseburger, Supersized French Fries, a large shake and an fired apple turnover? Did you wonder how all that fat got dissolved into your blood? … Try dissolving a stick of butter in a glass of water. Whenever it was, thank your LIPOPROTEINS! It’s your Lipoproteins that allow all the disgusting fat you eat to enter your plasma without “cloging” things up. Lipoproteins provide the transportation system for different types of water insoluble fatty crap Actually, sometimes things do get clogged up - It’s called a Heart Attack!!! We need to study lipoproteins because of their relationship to heart disease - The #1 cause of death in the United States

KEY TERMS Arteriosclerosis CAD Cholesterol Chylomicrons Endogenous lipoproteins Exogenous lipoproteins Fatty Acids Friedwald Calculation HDL LDL Lipoprotein Triglycerides VLDL Saturated Fatty Acids Unsaturated Fatty Acids “Good Cholesterol” “Bad Cholesterol” Lipid Lipemic Fasting blood specimen

List the reference ranges for the lipoproteins Objectives Define the general function, origin and structure of the lipoproteins Define the names, general composition and function of the following HDL LDL VLDL Chylomicrons Discuss the relationships between various lipoproteins and CAD List the reference ranges for the lipoproteins Discuss the basic test methodologies for lipoprotein testing

What are lipoproteins and why do we need them? Lipoproteins are a handful of different molecules that interact with water insoluble fat molecules, and transports those fats in the plasma The textbook describes the lipoproteins as “oil tankers” Different lipoproteins are responsible for the transportation of different fats Lipoproteins allow fat to be dissolved into the plasma

There are 4 types of lipoproteins Chylomicrons Transport of dietary triglycerides from the GI tract to the liver Very Low Density Lipoproteins ( VLDL ) Transport of triglycerides from the liver to tissues for storage and energy Low Density Lipoproteins ( LDL ) Transports cholesterol to peripheral tissues High Density Lipoproteins ( HDL ) Transports cholesterol away from the peripheral tissues to the liver

Lipid Chemistry Lipids include Cholesterol Triglycerides Phospholipids Glycolipids Lipids are important components of cell membranes

Fatty acids are short to long carbon chained molecules Saturated fatty acids No double bonds between carbons ( C – C ) Solid at room temperature Animal sources Unsaturated fatty acids Double bonds between carbon bonds ( C = C ) Usually liquid at room temp Plant sources

Triglycerides Glycerol with 3 attached fatty acids Exogenesis source : Dietary Endogeneous : Liver and tissue storage 95 % of body fat is triglycerides Energy source when plasma glucose is decreased Triglyceride catabolism is regulated by lipase, epinephrine and cortisol Triglycerides transported by Chylomicrons ( exogeneous ) and VLDL ( endogenous )

Transported by HDL and LDL Cholesterol Found only in animals Important component of membranes, steroid hormones, bile and Vitamin D Exogeneous cholesterol comes from diet Endogeneous cholesterol is synthesized by the liver 70 % of cholesterol associated with cellular components 30 % is in the plasma ( ⅓ free form , ⅔ esterfied ) Transported by HDL and LDL

Phospholipids Important components of cell membranes Lecithin and sphingomyelin are utilized to determine fetal lung maturity from amniotic fluid ( L / S Ratio ) Glycolipids Lipids with a carbohydrate component ABO antigen are glycolipids

Apoliproproteins Outer protein “shell” of the lipoprotein molecule The protein – lipid interaction allows the water insoluble lipid to become soluble in plasma The apolipoprotein is responsible for the interactions with cell membranes and enzymes to transport lipids to specific locations

Lipoproteins Classified according to density and electrophoresis migration Lipoproteins differ in their sizes, weights, chemical composition and artherogenicity ( association with heart disease ) There are four main types of lipoproteins CHYLOMICRONS VLDL LDL HDL

Chylomicrons Transports exogeneous ( dietary ) triglycerides 90 - 95 % by weight is triglycerides Absent from fasting plasma Removed from the plasma within 6 hours by the liver Inadequate clearance produces a creamy layer on the plasma VLDL ( Very Low Density Lipoproteins ) Transports endogeneous triglycerides from liver to tissues 50 - 65 % by weight is triglycerides Excess dietary carbohydrates are converted to triglycerides by the liver

LDL ( Low Density Lipoproteins ) Transports cholesterol from liver to the tissues Synthesized in the liver Approximately 50 % by weight cholesterol Most atherogenic lipoprotein … “ Bad Cholesterol “

HDL ( High Density Lipoprotein ) Transports excess cholesterol from the tissues back to the liver ( reverse transport ) Synthesized in the liver and intestines Composition 30% PHOSPHOLIPIDS 20% CHOLESTEROL 50% APOPROTEIN The “good “ cholesterol

Lipoprotein physiology and metabolism Water insoluble lipids are digested unto more water soluble ( polar ) compounds in the GI tract Triglycerides are digested into fatty acids Cholesterol – esters are converted into free cholesterol Fatty acids are converted into triglycerides by the liver anf adipose tissues ( fat ) Most cholesterol synthesis occurs in the liver - Most cholesterol lowering drugs target this synthesis Cholesterol is a main component of bile ( needed for dietary absorption of fat )

Exogeneous Pathway Transport of dietary lipids, mostly the chylomicrons transportation of triglycerides to the liver Endogeneous Pathway Transportation of lipids from the liver to the tissues ( VLDL & LDL )

Effects of hormones Insulin Remember, insulin always decreases plasma glucose Inactivates lipase … decreases lipolysis and the catabolism of triglycerides to fatty acids / glucose Stimulates lipogenesis ( fatty acid conversion to triglycerides ) Insulin helps make fat In diabetes mellitus, insulin deficiency promotes the release of fatty acids and their conversion to triglycerides by the liver

Lipoprotein analysis Laboratory measurement of the lipoproteins is important because of their association with Coronary Artery Disease ( CAD ) CAD is the # 1 cause of premature death in the United States Because of its connection with public health, lipid testing and interpretation has been standardized to provide consistent results that can be utilized for the risk assessment of CAD

Total Cholesterol Methodologies Cholesterol ester Free cholesterol Free cholesterol H2O2 H2O2 + Chromogen Colored Chromogen Cholesterol-Ester-Hydrolase Cholesterol Oxidase Peroxidase

Triglyceride testing Most methodologies utilize enzymes that break glycerol from its fatty acids Free glycerol is proportional to the initial triglyceride concentration

HDL ( High Density Lipoprotein ) testing Non – HDL is precipitated from plasma Dextran sulfate ( precipitating agent ) is added to patient’s plasma Non – HDL precipitates and is centrifuged to the bottom The supernate contains only HDL HDL in the supernate is measured using a conventional total cholesterol methodology Elevated triglycerides ( > 400 mg / dl ) may interfer with the separation of non – HDL Many labs will not perform HDL testing when triglyceride concentrations exceed 400 mg / dl

LDL ( Low Density Lipoprotein ) testing “Bad cholesterol “ … Artherogenic Direct measurement if LDL is uncommon because of technical difficulities Friedewald estimation ( calculation ) Test: Total Cholesterol, Total Triglycerides and HDL with routine procedure Estimate the LDL with the following : VLDL = Trig / 5

Disease prevention and treatment Arteriosclerosis #1 cause of death and disability Cholesterol deposits ( plaque ) in coronary arteries occlude blood flow Heart ( CAD ) Arms / Legs ( PVD ) Brain (CVD ) Plaque formation is associated with Increased plasma cholesterol Increased plasma LDL Decreased plasma HDL Diet, exercise and diabetes are major contributory factors of CAD

Target Ranges ( NECP ) Reference Ranges Total Cholesterol < 200 mg / dl High Density Cholesterol ( HDL ) > 35 mg / dl Low Density Cholesterol ( LDL ) < 130 mg / dl Triglycerides Reference Ranges Total Cholesterol 140 - 200 mg / dl HDL 30 - 75 mg / dl LDL 55 - 130 mg / dl Triglycerides 65 - 155 mg / dl

Other Risk Factors for CAD Gender – Males at higher risk Increased age Family history Genetic Abnormalities Stress – Type A personalities Hyperlipidemia Physical inactivity Hypertension Smoking Diabetes Elevated CRP Elevated homocysteine Elevated Lp(a)

Ultracentrifuge High plasma lipid concentrations can cause excessive plasma turbidity and interfere with spectrophotometric methods. Lipoproteins can be spun down in this special centrifuge. Plasma is placed inside the “donut.” Lipids spin to the outside of the donut.

Lipoprotein Top 10 Lipoproteins transport various lipids ( fats ) thru plasma to different locations High relationship between specific lipoproteins and CAD Apolioproteins are the protein “shells” that interact with lipids and allow them to be water soluble HDL ( Good Cholesterol ) transports cholesterol away from tissues to the liver LDL ( Bad Cholesterol ) transports cholesterol to the tissues from the liver VLDL transports endogenous triglycerides from liver to tissues Chylomicrons transport exogenous triglycerides from GI tract to liver Accurate Lipoprotein testing requires a fasting specimen LDL is not directly measured , but calculated from the Friedwald Calculation

Lipid Links http://www.indstate.edu/thcme/mwking/lipids.html http://www.nhlbi.nih.gov/chd/chdexp.htm http://www.fda.gov/fdac/features/1999/199_chol.html http://www.lifeclinic.com/focus/cholesterol/about_it.asp