Plasma Lipids in Animals

Plasma Lipids

Lipids are heterogenous molecules soluble in organic solvents but not in water. They are esters of fatty acids, formed by the reaction of  fatty acids with glycerol. Blood lipids (or blood fats) are lipids in the blood and they are present either free or bound to other molecules. Blood lipids are mainly fatty acids, triglycerides, lipoproteins and cholesterol.

Since lipids are insoluble in water they are mostly transported in a protein covering, and the density of the lipids and type of protein determines the fate of the lipid and its influence on metabolism. The concentration of blood lipids depends on dietary intake, absorption from the intestine and excretion and uptake and secretion from cells.

Fatty acids of Blood Plasma

Fatty acids are present in different forms {as in chylomicrons, Very low density lipoproteins (VLDL), Low density lipoproteins (LDL)} in blood. In addition, the fatty acids released from adipocytes exist in the blood as free fatty acids.

Short- and medium chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein. Long-chain fatty acids are too large to be directly released into the intestine capillaries. Instead, they are coated with cholesterol and protein (protein coat of lipoproteins) into a compound called as chylomicrons. The chylomicrons enter into lymphatic capillary and then into the bloodstream (having bypassed the liver).

The concentration of blood fatty acids increase temporarily after a meal which increases the uptake of fatty acids in different cells of the body like liver cells, adipocytes and muscle cells. This uptake is stimulated by insulin from the pancreas.

Some of the fatty acids taken up by the liver is converted into VLDL and again secreted into the blood.

When the concentration of fatty acids in the blood decreases, this triggers adipocytes to release stored fatty acids into the blood as free fatty acids, in order to supply the energy for the muscle cells and other cells.

Lipoproteins of Blood plasma

Lipoproteins are complex aggregates of lipids and proteins that increases the solubility of lipids and enable their transport throughout the body.

Lipoproteins are synthesised mainly in the liver and intestines.

The most abundant lipid constituents of lipoproteins are triacylglycerols, free cholesterol, cholesterol esters and phospholipids (phosphatidylcholine and sphingomyelin); fat-soluble vitamins and anti-oxidants are also transported in this way.

The lipoproteins contain different protein components called apoproteins (or apolipoproteins). Apoproteins are required to solubilise the non-polar lipids in the circulation. These proteins determine the overall structures and metabolism, and their uptake in liver and peripheral tissues.

Lipoproteins are classified as chylomicrons (CM), very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL), based on the relative densities on ultracentrifugation.

Based on the relative mobilities on electrophoresis, lipoproteins can also be classified into α, pre-β and β lipoproteins that correspond to HDL, VLDL and LDL, respectively.

Physical properties and lipid compositions of lipoprotein classes

Lipoproteins are spherical in shape with the core containing non-polar lipids, triacylglycerols and cholesterol esters, and a surface layer consisting of phospholipids and non-esterified cholesterol, which also serve to present a hydrophobic face to the aqueous phase.

The principal role of the chylomicrons and VLDL is to transport triacylglycerols as a source of fatty acids from the intestines or liver to the peripheral tissues.

Chylomicrons: They are largest of the lipoproteins; they are formed in the intestinal mucosa and transport triacylglycerols from the intestinal mucosa to the liver or to other tissues; they carry mostly fats in the form of triglycerides and cholesterol. In the liver, chylomicron particles release triglycerides and some cholesterol.

VLDL: They are synthesised in liver. The liver converts the excess fatty acids (dietary or synthesised) into very low density lipoproteins (VLDL) and secretes them into plasma; in the plasma they are converted to intermediate density lipoproteins(IDL), and then to low-density lipoprotein (LDL) particles. VLDL transports triacylglycerols from liver to peripheral tissues.

LDL: It is the main transporter of cholesterol to the peripheral tissues. When they are present in excess they are strongly associated with the formation of atheromatous (plaque formation within arterial wall) disease within the arteries. For this reason, LDL is referred to as “bad cholesterol”.

HDL: They are synthesised in liver. The HDL removes excess cholesterol from peripheral tissues and delivers them to the liver for excretion in bile in the form of bile acids, a process that has been termed ‘reverse cholesterol transport’, helps to lower blood cholesterol level and hence HDL is commonly called “good cholesterol”.

After being transported to the liver by HDL, cholesterol is delivered to the intestines via bile production. However, 92-97% is reabsorbed in the intestines and recycled via enterohepatic circulation.

Cholesterol in blood Plasma

It is lipid, waxy steroid found in the cell membranes and transported in the blood plasma of all animals.

It is an essential component of mammalian cell membranes where it is required to establish proper membrane permeability and fluidity.

In addition, cholesterol is an important precursor molecule for the biosynthesis of bile acids, and several fat soluble vitamins, Vitamin D₃ and the steroid hormones, including the adrenal gland hormones such as cortisol and aldosterone as well as the sex hormones like progesterone, estrogens, and testosterone and their derivatives.

Within the cell membrane, cholesterol also functions in intracellular transport, cell signalling and nerve conduction.

Since cholesterol is essential for life, it is primarily synthesized de novo, within the body, with smaller contributions from the diet.

Excessive levels of cholesterol in blood circulation however are strongly associated with progression of atherosclerosis.

The fate of cholesterol in the blood is highly determined by its constitution of lipoproteins – some types (LDL) favour transport towards body tissues and others (HDL) towards the liver for excretion into the intestines.

The average amount of blood cholesterol varies with age, typically rising gradually as age advances.

Hyperlipidemia refers to the elevated or abnormal levels of cholesterol and triacylglycerols in the blood. One of the most clinically relevant lipid substances is cholesterol, especially on atherosclerosis and cardiovascular disease.

Lipemia is a term used to refer marked hyperlipidemic condition resulting in plasma presenting a milky white appearance.

Postprandial hyperlipidemia is most common in animals consuming fatty diet; hence to evaluate lipid status, blood samples have to be taken from fasting animals.

Hyperlipidemia is noticed in dogs and man with hypothyroidism – main lipid increased is cholesterol. Pancreatitis, uncontrolled diabetes, cholestasis, hyperadrenocorticism are some other conditions causing hyperlipidemia.

Hypercholesterolemia is the presence of high levels of cholesterol in the blood. It is not a disease but a metabolic derangement that can be secondary to many diseases and can contribute to many forms of disease, most notably cardiovascular disease.

Sources of Cholesterol

Animal fats are complex mixtures of triglycerides with phospholipids and cholesterol; hence, all foods containing animal fat contain cholesterol to varying extents.

Cholesterol is not present in plant based food sources. However, plant products such as flax seeds and peanuts contain cholesterol-like compounds called phytosterols, which are suggested to help lower serum cholesterol levels.

About 20–25% of total daily cholesterol production occurs in the liver; other sites of high synthesis rates include the intestines, adrenal glands and reproductive organs.

Synthesis within the body starts with one molecule of acetyl CoA and one molecule of acetoacetyl-CoA. HMG-CoA reductase is an important enzyme in the cholesterol synthesis. Total cholesterol is the sum of HDL cholesterol, LDL cholesterol and 20% of triglycerides.