Lipoprotein interconversions during passage through subcutaneous adipose tissue in humans in vivo.

Adipose tissue lipoprotein lipase (LPL) is the enzyme responsible for hydrolysis of circulating lipoprotein triacylglycerol (TAG). Successive passages of very-low-density lipoprotein (VLDL) through adipose tissue are thought to produce progressive delipidation of the particles, with the eventual formation of intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL) [ 1 I . The hydrolysis of TAG is accompanied by the movement of other components between lipoprotein fractions. For instance, with the shrinkage of the particles, surface components including cholesterol and, in the case of chylomicrons, apolipoprotein (apo)-A1 may move into high-density lipoprotein (HDL) particles [ I ] . The evidence for the occurrence of these processes in vivo is still indirect. It is based mostly on the movement of labelled components between fractions, and on studies of lipoprotein lipase action in vitro. We have now studied the changes in lipoproteins during their passage through adipose tissue in vivo in humans, using a recently described technique for measurement of arteriovenous differences across these tissues [2].