Studies on the radioiodination of very low density lipoprotein obtained from different mammalian species

Very low density lipoprotein (VLDL) isolated from human, pig and rat serum showed marked differences in labelling patterns when radioiodinated under similar conditions, with greater amounts of ¹²⁵ I-labelled iodine being incorporated into the lipid moieties of pig and rat VLDL than into human VLDL. The effect (on their biological behaviour) of introducing increasing amounts of iodine into the lipoproteins was tested for both human and rat VLDL with preparations containing iodine:protein (I/P) ratios varying from < 0.5 to 12. After injection into rats and analysis of disappearance curves, only minor differences in catabolic rates were observed between different preparations. Thus, rat VLDL, with I/P ratios varying from < 0.5–12 showed fast and slow exponential components varying in half life ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) between 40–63 min and 10–11 h, respectively, while human VLDL with comparable I/P ratios, had a fast component varying from ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) 50–78 min and a slower component ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) of 10.5–12.5 h The plasma disappearance of human VLDL, labelled by the lactoperoxidase method, was also characterised by two exponential components with mean half lives of 67 min and 11 h, respectively, suggesting similarity in the biological behaviour of human VLDL labelled both enzymatically and with iodine monochloride. The results also indicated similarity in the catabolism of rat and human VLDL by the rat, suggesting that this animal is a suitable model for studying VLDL metabolism.