We have found that in vitro lipolysis of human very low density lipoproteins (VLDL) by purified bovine milk lipoprotein lipase (LpL) promotes degradation of the apolipoprotein (apo) B moiety of VLDL. Analysis by sodium dodecyl sulfate-polyacrylamide gradient gel electrophoresis showed that lipolysis of VLDL by purified LpL for 1 h at 37 degrees C induced the selective degradation of the high Mr apo-B (apo-B-100) from most hypertriglyceridemic VLDL and from a few normolipidemic VLDL into several small fragments with molecular weights ranging from 90,000-490,000. No detectable degradation of apo-B occurred in control VLDL when incubated without LpL. The apo-E moiety of VLDL from certain individuals was also degraded following lipolysis of VLDL, and the extent of degradation of apo-B and -E in VLDL was varied among the individual VLDL. The major degradation products of apo-E, identified from the gel, were 31,000- and/or 28,000-Da species. In contrast to the apo-E moiety of VLDL, purified apo-E was not degraded when incubated with LpL. Incubation of low density lipoproteins (LDL) with LpL showed only a minimal effect on the apoproteins of LDL. When high density lipoprotein (HDL) was included in the lipolysis mixture as an acceptor of lipolytic surface remnants, the apoproteins of HDL remained unaltered, while the apo-B moiety of VLDL remnants in the mixture was degraded. Inclusion of protease inhibitors in the lipolysis mixture prevented the degradation of apo-B, but the hydrolysis of VLDL-triglyceride was minimally affected. A selective degradation of apo-B in VLDL also occurred during lipolysis of VLDL when VLDL was perfused through rat hearts. These results suggest that conformational changes in apo-B and apo-E caused by VLDL lipolysis may increase the susceptibility of apo-B and apo-E to degradation by the proteases co-isolated with VLDL. The consequences of the lipolysis-induced degradation of apo-B and apo-E on changes in metabolic properties of VLDL remnants remain to be determined.