Evidence for the existence of only one triacylglycerol lipase of rat liver active at alkaline pH

A novel phospholipase A1 with sequence homology to a mammalian Sec23p-interacting protein, p125

p125, a mammalian Sec23p-interacting protein, exhibits sequence homology with bovine testis phosphatidic acid-preferring phospholipase A(1). In this study, we identified and characterized a new homologue of p125, KIAA0725p. KIAA0725p exhibited remarkable sequence similarity with p125 throughout the entire sequence determined but lacked an N-terminal proline-rich, Sec23p-interacting region. In vitro binding analysis showed that KIAA0725p does not bind to Sec23p. KIAA0725p possessed phospholipase A(1) activity preferentially for phosphatidic acid. We examined the effects of overexpression of KIAA0725p on the morphology of organelles. Overexpression of KIAA0725p, like that of p125, caused dispersion of the endoplasmic reticulum-Golgi intermediate compartment and Golgi apparatus. Different from the case of p125, overexpression of KIAA0725p resulted in dispersion of tethering proteins located in the Golgi region and caused aggregation of the endoplasmic reticulum. Our results indicate that KIAA0725p is a new member of the phosphatidic acid-preferring phospholipase A(1) protein family and suggest that the cellular function of KIAA0725p is different from that of p125.

Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within the lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg(2+). Incubating these fused microsomes with tri[(3)H] oleoylglycerol and [(14)C]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [(3)H]:[(14)C] ratio close to 2:1. The data suggest that the precursor tri[(3)H]oleoylglycerol was hydrolyzed by microsomal lipase to membrane-bound di[(3)H]oleoylglycerol and subsequently re-esterified with luminal [(14)C]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was inactive, is consistent with the existence of a latent diacylglycerol acyltransferase (DGAT II) within the microsomal lumen. Moreover, because luminal synthesis of TAG was carnitine-dependent and markedly reduced by glybenclamide, a potent carnitine acyltransferase inhibitor, microsomal carnitine acyltransferase appears to be essential for trafficking the [(14)C]oleoyl-CoA into the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymatic process leading to the synthesis of luminal triacylglycerol, which is a major component of very low density lipoproteins.

Synthesis and secretion of triacylglycerol lipase by cultured rat hepatocytes

Rat hepatocytes isolated by collagenase perfusion were cultured for 48-72 h and examined for synthesis and secretion of hepatic triacylglycerol lipase activity. Low levels of enzyme activity found in the culture medium increased with time of incubation, and a 3-10-fold rise was encountered in the presence of optimal concentrations of heparin (5 U/ml). After interruption of enzyme synthesis by cycloheximide, plateauing of enzyme activity in the medium occurred, indicating that addition of heparin may not only stabilize but also enhance hepatic triacylglycerol lipase secretion. Synthesis and secretion of hepatic triacylglycerol lipase was not related to cell density, and enzyme secretion was encountered in subconfluent cultures. Release of enzyme activity into the medium was not sensitive to chlorpromazine, a lysosomal enzyme inhibitor, but was completely inhibited by treatment with tunicamycin, an inhibitor of glycosylation. As release of enzyme activity could be maintained for 12 h in the absence of serum, possible hormonal regulation was sought. Under the present experimental conditions, no modulation of hepatic triacylglycerol lipase was encountered by either gonadal or thyroid hormones. Addition of cyclic AMP to the culture medium resulted in a 30% decrease in enzyme activity. The dependence of hepatic triacylglycerol lipase secretion on the intactness of the Golgi apparatus and on vesicular transport was demonstrated by the treatment with monensin. The present results show that cultured rat hepatocytes provide a good model system by which the regulation of synthesis and secretion of hepatic triacylglycerol lipase can be studied.

Salt-resistant (hepatic) lipase. Evidence for its presence in bovine liver and adrenal cortex

Alkaline lipolytic activities in bovine liver and adrenal cortex have been investigated; each tissue has a salt-resistant, hepatic-type lipase activity of which we describe a partial purification. Properties of the partially purified enzymes have been compared directly with those of authentic hepatic lipase prepared from rat liver. Furthermore, a similar activity has been detected in bovine post-heparin plasma. These findings contrast with a previous report that bovine post-heparin plasma and liver extracts lack hepatic salt-resistant lipase.

Characteristics of multiple forms of the acidic triacylglycerol lipase(s) of canine cardiac myocytes

Acidic lipase activity was extracted by digitonin treatment from particulate fractions prepared from isolated adult canine myocytes. Both methylumbelliferyloleate (MUO) and trioleoylglycerol were hydrolyzed with an apparent Km of 13 and 135 microM, respectively. The primary products of trioleoylglycerol lipolysis were oleic acid and 1,2-dioleoylglycerol. Hydrolysis of either MUO or triacylglycerol was stimulated in vitro by the addition of cardiolipin or Triton X-100. Triton X-100 alone was sufficient for maximal stimulation of MUO hydrolysis, but cardiolipin further stimulated triacylglycerol lipolysis in the presence of an optimal concentration of Triton X-100. Cardiolipin increased the Vmax without altering the Km for trioleoylglycerol. Upon gel filtration chromatography the 4-methylumbelliferyloleate and triacylglycerol lipase activities eluted in regions consistent with molecular weights of approx. 47 000 and 55 000, respectively. Chromatofocusing revealed predominantly one form of acidic 4-methylumbelliferyloleate hydrolase (pI approx. 6.3), whereas acidic triacylglycerol lipase activity eluted continuously in the pH gradient from 7.2 to 4.3 with no clearly predominant peak of activity. Two forms of both 4-methylumbelliferyloleate and triacylglycerol lipase were eluted from columns of carboxymethyl Bio-Gel at pH 5.7; one form of each lipase activity was not bound and another form of each lipase was eluted with 50-60 mM KCl. The non-bound forms of each lipase were indistinguishable from their respective carboxymethyl-bound forms on the basis of pH dependency or kinetically (similar Km). The non-bound and carboxymethyl-bound peaks of lipolytic activity differed in the ratios of 4-methylumbelliferyloleate hydrolase to triacylglycerol lipase activity. The results suggest that the cardiac myocyte contains multiple forms of acidic lipase, and that the catalytic units primarily responsible for the hydrolysis of methylumbelliferyl esters and triacylglycerols may not be identical.