Radioiodination of human low density lipoprotein: a comparison of four methods.

A comparison has been made of four labelling techniques used to radioiodinate human low density lipoprotein (LDL). (1) Chloramine T iodination at pH 7.4 was 20-25% efficient and gave a product immunologically indistinguishable from native LDL. Approximately 30% of the incorporated radioactivity, however, was found in LDL lipids, and the metabolic decay of the labelled complex in rats did not obey first order kinetics. Radiolabelling at pH 10 reduced the uptake of 125I into lipids to 10% but also cut the overall incorporation of radioiodine by a factor of 7. (2) Lactoperoxidase labelling and (3) conjugation with iodinated N-succinimidyl-3-(4-hydroxyphenyl)propionate were highly efficient (100%), but incorporation of radioactivity into the lipid moiety was unacceptably high (approximately 30%). (4) The efficiency of iodine monochloride labelling was highly reproducible and the product was immunologically indistinguishable from native LDL. Incorporation of radioactivity into the lipid moiety was less than 4% when the I/protein ratio of the product was kept at or below 1 : 1. Decay of the radiolabelled LDL in rats was monoexponential.