Very low density lipoproteins of rabbits fed cholesterol

Abstract

Analytical ultracentrifugation has been employed to investigate the lipoprotein spectra of rabbits fed cholesterol and humans with type III hyperlipoproteinaemia. The results show considerable similarities between the lipoprotein spectra of the two, whether studied individually or after being averaged. Despite this similarity in spectra, the rabbit fed cholesterol was not considered to be a suitable experimental model for human type III hyperlipoproteinaemia as the lipoproteins probably have a different metabolic origin.

On feeding cholesterol to rabbits, two peaks of lipoproteins were identified within the very low density lipoprotein (VLDL) region of Sf 12 to 400. The peak of smaller lipoproteins floated between Sf 12 and 60 and are a well known component of the serum of rabbits fed cholesterol. The larger lipoproteins, floating at Sf 270, have not been reported before. The greater part of this thesis is concerned with the study of the similarities of, and differences between, these two populations of lipoproteins.

Rabbits were fed 0.8% cholesterol in their diet for two weeks and the two peaks of VLDL isolated from serum by preparative ultracentrifugation. The smaller lipoproteins had a peak Sf averaging 37, mean diameter of 36 nm, density of 1.00 g/ml, and their chemical composition agreed closely with previous reports. The larger particles had a peak Sf averaging 270, mean diameter of 80 nm, density of 0.97 g/ml and a high (80%) cholesterol ester content and low (4%) triglyceride content. The fatty acid composition of the cholesterol esters, phospholipids and triglycerides was similar in both fractions.

It was proposed that the two particles were both remnant chylomicrons as all the differences seen between the two particles could be accounted for by their difference in size. They were named small (SCR) and large chylomicron remnants (LCR).

Metabolic studies on the LCR and SCR using radioactive tracers confirmed that they both originated from the intestines, and indicated that the main site of catabolism of these particles was the liver. On injecting LCR and SCR, labelled in their cholesterol moiety, into recipient rabbits fed normal or cholesterol diets, it was observed that LCR disappeared from the plasma faster in the normal animals, but SCR was removed more rapidly in the rabbits fed cholesterol. An hypothesis was proposed to explain this difference in catabolism.

The large number of samples that were subjected to analytical ultracentrifugation necessitated the efficient calculation of the results from this machine. As the manual method first set up for the job was found to be rather tedious and clumsy, a computer program was written. This meant that all necessary corrections could be incorporated and output could be in the form of a convenient and useful spectrum. Continuous development of this program throughout the duration of this work has produced a simple but useful and versatile program.