Measurement of receptor-independent lipoprotein catabolism using 1,2 cyclohexanedione-modified low density lipoprotein.

The utility of 1,2 cyclohexanedione-modified low density lipoprotein (CHD-LDL) as a marker for the measurement of receptor independent LDL catabolism has been assessed by examining its metabolic properties in cultured human fibroblasts and in rabbits. Cell culture studies showed that the inhibition of high affinity membrane receptor binding produced by the modification could be partially reversed by prolonged incubation of the CHD-LDL at 37 degrees C. Pre-exposure of the complex to alkaline pH (pH 10.5) prevented this and yielded a product that was apparently stable. Despite its regained ability to bind to the fibroblast receptor, 125I-labeled CHD-LDL incubated at 37 degrees C for 24 hr either in vivo or in vitro was removed from rabbit plasma in the same manner as freshly prepared 131I-labeled CHD-LDL and as 131I-labeled CHD-LDL that had been treated at pH 10.5. However, its plasma clearance was significantly faster than that of reductively methylated LDL. We believe that this may result from differential catabolism of these modified lipoproteins rather than from susceptibility of the CHD-LDL to receptor-directed catabolism.