In vitro labeling of beta-apolipoprotein with 3H or 14C and preliminary application to turnover studies

3-H- or 14-C-labeled methyl groups were introduced into apolipoproteins of human and pig low density lipoproteins (LDL). 98% of the label was recovered in the apoprotein of radiomethylated LDL. Such methylated lipoprotein was compared with the corresponding unlabeled LDL with respect to its electrophoretic and immunochemical properties, and its behavior in the analytical ultracentrifuge. The data demonstrated that neither the human nor pig LDL underwent gross changes as a result of methylation. The applicability of radiomethylated pig LDL as a tracer for studying the turnover of LDL in pigs was examined. The results showed that the behavior of unscreened and screened 3-H-labeled LDL was similar. The LDL disappeared with an initial t1/2 of 1.1 hr and a later t1/2 of 30 hr. These values agreed well with those reported for radioiodinated LDL. The technique of radiomethylation of lipoprotein may afford an advantage over radioiodination as it may label peptides that do not have tyrosine.