WHAT IS A LIPOPROTEIN?

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WHAT ON EARTH IS A LIPOPROTEIN?

 

 

By Malcolm Kendrick MbChB, MRCGP (email - malcolm )

 

I have written a few columns on heart disease for Red Flags and the response

has been very positive. However, there is a major problem that emerges quite

clearly from e-mails that I get back. The problem is that there is an

enormous level of confusion about the whole area of cholesterol, lipids,

lipoproteins, fats etc. So I thought I should provide a simple primer on

this area, as it makes debate and discussion a lot easier.

 

Before getting into the area, I must admit that I have a great deal of

sympathy with the confusion. When I first started looking at the

diet-heart/cholesterol hypothesis I found the science to be almost totally

incomprehensible, and much of this is due to, what I refer to, as

terminological inexactitude.

 

To provide a couple of simple examples. A high level of low density

lipoprotein (LDL) in the blood is usually referred to as a high cholesterol

level. A high level of very low density lipoprotein (VLDL) in the blood is

usually referred to as a high triglyceride level. Frankly this is nuts, as

LDL and VLDL contain both triglycerides and cholesterol - and neither

triglycerides or cholesterol float free in the blood.

 

Let's try another example. An LDL with a protein attached to it called

apolipoprotein b-100 is called LDL. LDL with a protein attached to it called

apolipoprotein (a) is called Lipoprotein (a). Or Lp(a).. I sense confusion

arising.

 

So, let's start at the very beginning, it's a very good place to start.

Point number one. cholesterol is not a fat; it is called many different

things, even an alcohol, but one thing it is not, is a fat, or a fatty acid.

(Fats and fatty acids are the same thing, by the way). Nor can you make

cholesterol from fats.

 

Cholesterol starts life as a chemical called Acetyl coenzyme a. A relatively

ubiquitous building block that is used to make all sorts of things that the

body needs. The vast majority of cholesterol in your body is synthesised by

the liver from Acetyl coenzyme a. You only get about a quarter of your

cholesterol from dietary sources.

 

Point number two: Triglycerides are three fat molecules stuck to a Glycerol

molecule - which is where the tri and the glyceride come from. Although the

fat part seems to have gone missing in the nomenclature. Most fats are

transported around the body and stored as triglycerides.

 

When you eat cholesterol and/or fat, they are absorbed by the gut. But

neither fat/triglyceride, nor cholesterol can be dissolved in blood - they

are insoluble in water. So, they have to be wrapped up in a sphere known as

a lipoprotein in order to transport them out of the gut.

 

Point three: Lipoproteins come in many sizes. The biggest is a chylomicron

and the smallest is a high density lipoprotein (HDL). If a chylomicron were

the size of a football (soccer ball), a VLDL would be the size of a

baseball, an LDL would be the size of a golf ball, and an HDL the size of a

pea, perhaps even a petit pois.

 

All lipoproteins contain cholesterol and triglyceride - in varying

proportions. The basic function of a lipoprotein is to carry triglycerides

from the gut, or the liver, to fat cells, where the triglyceride is then

stored and used for energy when needed - in situations such as pressing the

remote control for the television, or chewing a hamburger.

 

Lipoproteins also transport cholesterol and triglycerides to the liver. When

a chylomicron reaches the liver, from the gut, it is grabbed, absorbed, and

then smashed to pieces. The liver then reconstructs the component parts into

VLDLs and sends them out into the bloodstream with an apolipoprotein b-100

protein stuck to the side.

 

As a VLDL travels around the body, fat cells snatch at it, chop bits off and

it gets smaller and smaller, turning first into an intermediate density

lipoprotein (IDL), then a low density lipoprotein (LDL). Once the

lipoprotein has reached LDL size, it is either re-absorbed by the liver and

re-used, or it is absorbed by other cells around the body that are in need

of cholesterol.

 

The reason why LDL can be absorbed is all to do with the apolipoprotein

b-100. This is the protein 'key' that the cells recognise. It is the key

that fits exactly into the LDL receptor on the cell wall. Once the b-100

molecule locks to the receptor, the receptor closes around the LDL, draws it

into the cell where the LDL is broken down into its component parts.

 

And what of High Density Lipoprotein (HDL)? HDL is not part of the same

metabolic 'loop' as the other forms of lipoproteins. It is made separately,

and appears to act as a cholesterol mop, scavenging loose cholesterol from

broken down cells and suchlike, and transporting it back to the liver. Which

is why it is often called 'good' cholesterol. It is called this even though

HDL isn't cholesterol, and cannot possibly have any effect on removing

cholesterol deposits from arterial walls. In short, it is neither

cholesterol, nor good. Apart from that it is a magnificently accurate form

of nomenclature.

 

Anyway. In short, lipoproteins are the 'taxis' that are used to transport

insoluble cholesterol and triglyercides around the body. Apart from HDL,

lipoproteins start big, as chylomicrons, and gradually get smaller as they

lose triglyceride. The VLDLs, produced by the liver get smaller and smaller

until they become LDLs. At which point they are reabsorbed into the liver,

or other cells.

 

What then, is the cholesterol level?

 

Well, it should be obvious by now that the cholesterol level doesn't

actually exist. For there is no cholesterol free in your bloodstream. You

can have a level of LDL, or VLDL, or chylomicrons, but you can't have a

level of cholesterol. And so all measurements of 'cholesterol' are actually

measurement of lipoproteins - which is where most of the terminological

confusion arises.

 

Thus, when someone uses the term 'total' cholesterol, what they mean is the

level of LDL, plus HDL, plus a few other lipoproteins e.g. Lipoprotein (a)

and/or some intermediate density lipoproteins that aren't quite LDLs, but

get mixed up in the process.

 

When the term LDL/cholesterol, or 'bad' cholesterol level is used, this

refers only to the level of LDL. This is usually about two thirds the level

of 'total' cholesterol. Other laboratories will tell you both the LDL and

HDL (good cholesterol) level, and give you the proportion of LDL to HDL.

With a ratio greater than three seen as 'bad' and a ratio less than three as

'good.' This can be presented as LDL:HDL 3.2:1, or whatever.

 

Some people think the level of VLDL is important, and they will give you

this measurement as well. But they will call it the level of triglyceride.

 

I think that is enough for one article. I hope that you find it a helpful

dash through the nomenclature used in this area. If I get some positive

feedback I could explain the difference between saturated fats and

unsaturated fats, and what the terms Omega 3 and Omega 6 actually mean, and

a few other things as well.