Binding of lipoprotein lipase to alpha 2-macroglobulin

The interaction between bovine lipoprotein lipase (bLPL) and human alpha 2-macroglobulin (alpha 2M) was studied by use of non-denaturing PAGE and gel-permeation, Zn(2+)-Sepharose and heparin-Sepharose chromatography. It was demonstrated that bLPL in vitro binds non-covalently to native alpha 2M, but not to the receptor-recognized form produced by treatment of alpha 2M with chymotrypsin or methylamine. A small amount of bLPL was bound covalently to alpha 2M by disulphide interchange, when incubated together with chymotrypsin or methylamine. Whereas alpha 2M in complex with bLPL still bound to Zn(2+)-Sepharose, bLPL lost the ability to bind to heparin-Sepharose. Preincubation of bLPL with heparin prevented complex-formation with alpha 2M, suggesting that alpha 2M interacts with the heparin-binding domain of bLPL. Experiments in which 125I-bLPL was incubated with human plasma at 20 degrees C demonstrated an 11-17% binding of the labelled lipase to alpha 2M, indicating that this interaction may be of physiological significance.