Dataset Information

Rat liver contains a limited number of binding sites for hepatic lipase.

ABSTRACT: The binding of hepatic lipase to rat liver was studied in an ex vivo perfusion model. The livers were perfused with media containing partially purified rat hepatic lipase or bovine milk lipoprotein lipase. The activity of the enzymes was determined in the perfusion media before and after passage through the liver. During perfusion with a hepatic-lipase-containing medium the lipase activity in the medium did not change, indicating that there was no net binding of lipase by the liver. In contrast, more than 80% of the lipoprotein lipase was removed from the medium. This lipoprotein lipase activity could be recovered into the perfusion medium completely by heparin perfusion of the liver. If livers, first depleted of hepatic lipase by heparin, were subsequent perfused with a hepatic-lipase-containing medium, 90 +/- 24 m-units of the lipase activity was bound per g of liver (up to 1000 m-units/total liver). However, heparin treatment of the liver decreases the ability of the liver to re-bind hepatic lipase by 80%. Perfusion of rat livers with 0.3 M NaCl released 60% of the lipase activity into the medium. Upon subsequent perfusion of these livers with hepatic-lipase-containing media, 541 +/- 164 m-units of hepatic lipase could be bound per g of liver (up to 5000 m-units/total liver). The binding of hepatic lipase was also studied in livers of corticotropin (ACTH)-pre-treated rats. In these rats also, hepatic lipase bound only to livers which had been pre-perfused with heparin or 0.3 M NaCl. After heparin pre-perfusion, 88 +/- 12 m-units of hepatic lipase could be bound per g of liver, similar to that with livers of control rats not treated with ACTH. After prior salt perfusion, however, the capacity of the livers of ACTH-pre-treated rats to bind hepatic lipase was 212 +/- 60 m-units/g of liver. This is less than in livers of control rats (541 +/- 164 m-units/g of liver). These results indicate that in rat liver the binding of hepatic lipase is heterogeneous in character and consists of heparin-resistant and heparin-sensitive components. The hepatic-lipase binding capacity of the liver is saturable and fully utilized under various conditions. The heparin-sensitive binding capacity is lowered in ACTH-treated rats, whereas the heparin-resistant binding is unaffected. We postulate that the functional hepatic lipase activity can be regulated by changes in the binding capacity of the liver.

SUBMITTER: Schoonderwoerd K

PROVIDER: S-EPMC1137290 | biostudies-other | 1994 Sep

REPOSITORIES: biostudies-other

ACCESS DATA

Similar Datasets

Project description:The annexins are a multigene family of Ca(2+)-dependent phospholipid-binding proteins which contain novel types of Ca2+ sites. Using site-directed mutagenesis, we generated mutant proteins that show defects in the Ca(2+)-binding sites in a particular member of this family, the src tyrosine kinase substrate annexin II. Analysis of the relative Ca(2+)-binding affinities of annexin II mutants in a combined Ca2+/phospholipid-binding assay revealed two distinct types of Ca(2+)-binding sites. Three so-called type II sites are found in annexin repeats 2, 3 and 4 respectively. Two so-called type III sites are located in the first repeat and involve the glutamic acid residues at positions 52 and 95. Both types of sites were recently identified by X-ray crystallography in annexins V and I [Huber, Schneider, Mayr, Römisch and Paques (1990) FEBS Lett. 275, 15-21; Weng, Luecke, Song, Kang, Kim and Huber (1993) Protein Sci. 2, 448-458], indicating that similar principles govern Ca2+ binding to annexins in crystals and in solution. The two types of Ca(2+)-binding sites differ not only in their architecture but also in their affinity for the bivalent cation. The Ca2+ concentration needed for half-maximal phosphatidylserine binding is 5-10 microM for an annexin II derivative with intact type II but defective type III sites (TM annexin II) whereas a mutant protein containing defective type II but unaltered type III sites (CM annexin II) requires 200-300 microM Ca2+ for the same activity. Annexin II mutants with defects in the type II and/or type III sites also show different subcellular distributions. When expressed transiently in HeLa cells, TM annexin II acquires the typical location in the cortical cytoskeleton observed for the wild-type molecule. In contrast, CM annexin II remains essentially cytosolic, as does a mutant protein containing defects in both type II and type III Ca(2+)-binding sites (TCM annexin II). This indicates that the intracellular association of annexin II with the submembraneous cytoskeleton depends only on the occupation of type II Ca(2+)-binding sites.

Dataset Information

Rat liver contains a limited number of binding sites for hepatic lipase.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets