Immunochemical properties of lipoprotein lipase. Development of an immunoassay applicable to several mammalian species.

The reaction of bovine lipoprotein lipase with its antibodies was found to be conformation-dependent. One aspect of this was that most antisera were more reactive with denatured than with native 125I-labeled lipoprotein lipase. Another aspect was that denatured lipase did not compete effectively with native lipase for those antibodies which caused inhibition of the enzyme's activity. This latter observation leads to the conclusion that the inhibiting antibodies recognize conformation-dependent determinants on the native enzyme. Fab fragments prepared from an inhibiting antiserum blocked the binding of the lipase to triacylglycerol/phospholipid droplets. This suggests that the inhibition results from reaction of the antibodies with the enzyme as it exists in solution, either covering the lipid-binding site on the enzyme or making it impossible for the enzyme to go through the conformational transitions necessary for binding to lipid. Most rabbit antisera did not react with rat or mouse lipoprotein lipase, but some sera showed a weak cross-reaction. Antisera raised in hens showed a much stronger cross-reaction, enough to be useful for heterologous immunoassays. An immunoassay for the bovine lipase was developed. For reproducible results it was necessary to have tracer, standard and samples in denatured form. This was accomplished by heating them in SDS, and running the immunoreaction in a Triton X-100-containing medium.