Medium-chain vs long-chain triacylglycerol emulsion hydrolysis by lipoprotein lipase and hepatic lipase: implications for the mechanisms of lipase action

Abstract

To explore how enzyme affinities and enzyme activities regulate hydrolysis of water-insoluble substrates, we compared hydrolysis of phospholipid-stabilized emulsions of medium-chain (MCT) versus long-chain triacylglycerols (LCT). Because substrate solubility at the emulsion surface might modulate rates of hydrolysis, the ability of egg yolk phosphatidylcholine to solubilize MCT was examined by NMR spectroscopy. Chemical shift measurements showed that 11 mol % of [13C]carbonyl enriched trioctanoin was incorporated into phospholipid vesicles as a surface component. Similar methods with [13C]triolein showed a maximum solubility in phospholipid bilayers of 3 mol % (Hamilton & Small, 1981). Line widths of trioctanoin surface peaks were half that of LCT, and relaxation times, T1, were also shorter for trioctanoin, showing greater mobility for MCT in phospholipid. In assessing the effects of these differences in solubility on lipolysis, we found that both purified bovine milk lipoprotein lipase and human hepatic lipase hydrolyzed MCT at rates at least 2-fold higher than for LCT. With increasing concentrations of MCT, saturation was not reached, indicating low affinities of lipase for MCT emulsions, but with LCT emulsion incubated with lipoprotein lipase, saturation was reached at relatively low concentration, demonstrating higher affinity of lipase for LCT emulsions. Differences in affinity were also demonstrated in mixed incubations where increasing amounts of LCT emulsion resulted in decreased hydrolysis of MCT emulsions. Increasing MCT emulsion amounts had little or no effect on LCT emulsion hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the Lipid Emulsion on an Injectable Formulation of Lipophilic KW-3902, a Newly Synthesized Adenosine A1-Receptor Antagonist.

“…23,39) This would be explained by difference in the affinity for a series of apolipoproteins [40][41][42][43][44][45] in blood or the subsequent metabolism by the lipoprotein lipase. 37,[46][47][48] Since it has become apparent that a considerable amount of liposome-like fractions exist in lipid emulsion suspensions, we compared the pharmacokinetics and protein binding ability of em-KW-3902 or lipo-KW-3902 with that of free-KW-3902 directly. Em-KW-3902 showed the similar pharmacokinetic profile of free-KW-3902 (Fig.…”

Human Hepatic and Lipoprotein Lipase: The Loop Covering the Catalytic Site Mediates Lipase Substrate Specificity

“…Further analysis of the lipase lids reveals that the LPL lid contains four acidic and three basic amino acids, whereas the lid of HL contains only one acidic but five basic residues. In the investigation of substrate specificity, we controlled for different fatty acid preferences between LPL and HL (22)(23)(24)(25) by using oleic acid as the fatty acid in triglyceride (triolein) and phospholipid (DOPC) substrates. Thus, the observed differences in substrate specificity may, in part, be due to different abilities of the LPL and HL lids to accommodate the polar head group of phospholipids.…”

“…The two enzymes also differ in their substrate specificity, demonstrating preferences in their fatty acid positional specificity (20,21), fatty acid chain length (22,23), and degree of fatty acid saturation (24,25). In addition, LPL and HL differ in their preferred lipoprotein substrates.…”

Integral apolipoproteins increase surface-located triacylglycerol in intact native apoB-100-containing lipoproteins

“…Tremendous advances in genetic manipulation and protein chemistry continue to drive rapid progress in these areas. Studies with model triacylglycerol (TAG)-rich lipoprotein particles show that metabolism also depends on surface lipid composition (1,2). Paradoxical outcomes, such as increased apparent lipase activity with low-affinity substrates, occur when only whole particle lipid substrate composition is considered (2).…”

“…Studies with model triacylglycerol (TAG)-rich lipoprotein particles show that metabolism also depends on surface lipid composition (1,2). Paradoxical outcomes, such as increased apparent lipase activity with low-affinity substrates, occur when only whole particle lipid substrate composition is considered (2). This occurs presumably because most reactions involving nonpolar lipids take place at the interface between the particle and the surrounding aqueous medium; thus, both the affinity of the enzyme for the interface and the specific accessibility of the substrate molecules to the lipase drive reaction kinetics, rather than total particle lipid content per se (2).…”