Effects of calcitonin gene-related peptide and amylin on human osteoblast-like cells proliferation

Expression of mRNA for calcitonin gene-related peptide (CGRP) and CGRP receptor has been detected in osteoblasts indicating that CGRP could play a role in bone metabolism. In the present study, we evaluated the effect of CGRP on primary culture of human osteoblast-like cells proliferation. The peptide was able to stimulate [3H]thymidine incorporation in human osteoblast-like cells with a maximal effect at 10(-8) M. The proliferating activity of CGRP was not inhibited by the two antagonists, CGRP-(8-37) or amylin-(8-37), whereas amylin fragment antagonized the proliferating activity of amylin. In human osteoblast-like cells CGRP, but not amylin, was able to stimulate adenylyl cyclase activity and this effect was completely antagonized only by CGRP-(8-37) and not by amylin-(8-37). These data suggest that the CGRP induced stimulation of cAMP is not involved in the peptide proliferating effect in human osteoblast-like cells and that in this cell population there are receptor subtypes for CGRP, distinct from that of amylin.

The influence of sex and gonadectomy on the growth hormone and corticosterone response to hexarelin in the rat

The present study is designed to investigate the role of sex and gonadal status in the growth hormone (GH) and corticosterone response to hexarelin (HEXA), a GH-releasing peptide, which also causes a stimulatory action on the hypothalamic-pituitary-adrenal (HPA) axis. HEXA (80 microg/Kg) was administered intracarotid to anesthetized intact or gonadectomized male (ORC) and female (OVX) middle-aged rats. The GH stimulatory response to HEXA was gender-related since the GH increase was significantly (p < 0.001) higher in intact males (area under the curve, AUC= 12560 +/- 1784 ng/ml.45 min) than in females (AUC= 4628 +/- 257 ng/ml.45 min). This sex difference does not depend on circulating gonadal steroids since it persists in ORC (AUC = 11980 +/- 1136 ng/ml.45 min) and OVX (AUC = 5539 +/- 614 ng/ml.45 min) rats. The different effects of HEXA on corticosterone secretion detected in male and female rats are probably dependent on the prevailing activity of the HPA axis. In fact, in male rats that have low basal corticosterone levels, HEXA caused an increase in corticosterone secretion, which was significantly (p< 0.05) higher in ORC than in intact rats. The increase in corticosterone secretion by HEXA both in intact and OVX females was delayed, probably due to the elevated initial corticosterone levels, which could have activated the glucocorticoid negative feedback. We suggest that gender-specific patterns in the regulation of the hypothalamus-pituitary function could be responsible for the GH and corticosterone sexually differentiated responses to HEXA.

Evaluation of a fully automated assay to measure C-telopeptide of type I collagen in serum

The measurement of the degradation product of the C-telopeptide of type I collagen in serum (s-betaCTX) has been evaluated in the form of an automated chemiluminometric assay. The between-day coefficients of variation ranged between 2.6 and 6.0% over the concentration range 0.12 to 3.25 microg/l. The s-betaCTX value correlated well with alternative serum C-telopeptide (r = 0.983) and urine deoxypyridinoline (r = 0.729) measurements. A significant inverse correlation was found between the s-betaCTX measurement and creatinine clearance in patients with renal disease.

Determination of decision limits for ACS:systems cardiac troponin I

The availability of specific markers of myocardial damage has focused attention on the decision limits to be used in clinical practice. These markers can be employed in the diagnosis of myocardial infarction and risk stratification in patients with acute coronary syndromes but no infarction. The measurement of troponin I was undertaken in two 60 patient cohorts with these disease classifications. The best decision level for patients with acute myocardial infarction was found to be 3.1 microg/l; for the detection of minimal myocardial damage the cut-off was equal to the detection limit of the method (0.2 microg/l).

Multicenter evaluation of five assays for myoglobin determination

BACKGROUND

Lacking assay standardization, different myoglobin methods may produce results that differ significantly.

METHODS

A multicenter study was carried out to compare the analytical performance of five commercially available assays for myoglobin measurement. Linearity, imprecision, interferences, and method comparison were studied according to NCCLS guidelines, whereas reference values were determined following IFCC recommendations.

RESULTS

The BNA and Opus showed relatively high imprecision (all but one total CV >7.4%). Other assays showed lower CVs, but they varied among laboratories, particularly at a normal myoglobin concentration (Access, 6.0-11%; Hitachi, 3.8-5.8%; Stratus, 3.4-6.5%). Results were lower in anticoagulated samples on the Access, in heparin and citrate samples on the Stratus, and in citrate samples on the BNA and Opus, and increased in heparin and EDTA samples on the Hitachi. Use of separator gel produced results significantly lower (P <0.001) on the Hitachi and higher (P = 0.016) on the Opus. Bilirubin, turbidity, and hemoglobin had no effect on evaluated methods, but rheumatoid factor affected the Access. In method comparisons, high correlation coefficients (>/=0.98) were obtained. The Stratus gave higher results; however, the Access and BNA gave the lowest. The following upper reference limits (microgram/L) for men and women, respectively, were obtained: Access, 70 and 52; BNA, 51 and 49; Hitachi, 67 and 58; Opus, 80 and 50; and Stratus, 86 and 63.

CONCLUSION

The possibility of high imprecision and marked disagreement among commercial myoglobin assays should be carefully considered in clinical practice.

The role of sensory neurons in the antiulcer effect of centrally injected amylin in rat

Central administration of amylin (2.2 microg/rat, i.c.v.) reduces (from a minimum of 67% to 83%) indomethacin (Indo, 20 mg Kg(-1), orally) induced ulcers in rats. The anti-ulcer effect of the peptide is not removed by the administration of prokinetic drugs like domperidone or neostigmine but it is reduced by 35% in rats treated with capsaicin or with the CGRP antagonist, CGRP(8-37). These data indicate that amylin gastroprotection involves capsaicin-sensitive nerve fiber leading to CGRP-dependent gastric vasodilatory effect. Additional mechanisms could involve noradrenergic alpha(2) receptors as the peptide gastroprotective activity is reduced from 67% to 20% by the alpha(2) antagonist yohimbine.

Difference in substrate specificity between human and mouse lysosomal acid lipase: low affinity for cholesteryl ester in mouse lysosomal acid lipase

Lysosomal acid lipase (LAL) is essential for the intracellular degradation of cholesteryl esters (CE) and triacylglycerols (TG) that are delivered to lysosomes by low density lipoprotein (LDL) receptor mediated endocytosis. We have analysed the difference in the catalytic properties and substrate specificity of human and mouse LALs. LAL activities were measured in human and mouse fibroblasts and in HeLa cells transiently expressing wild-type or site-directed mutant LALs of the two species using the T7 vaccinia system. Cholesteryl esterase and triacylglycerol lipase activities were determined in cellular homogenates with a phospholipid/detergent vesicle assay, an assay frequently used to diagnose human LAL deficiency syndromes, and with LDL particles, a more physiological substrate. Characterisation of human and mouse LAL using these two assays demonstrated marked differences in their TG and CE hydrolysing activities. Compared to human LAL mouse LAL showed a much lower cholesteryl esterase activity in both assays used. The difference was more pronounced in the vesicle assay. The lower cholesteryl esterase activity of mouse LAL did not affect the LDL-CE degradation in intact fibroblasts. The analysis of site-directed mutants suggests a role of the non-conserved cysteine residue at position 240 in cholesteryl esterase activity in human LAL. Our results show a significant difference between human and mouse LAL in their specificity toward cholesteryl esters. The low cholesteryl esterase activity does not result in reduced LDL-cholesterol ester degradation in mouse fibroblasts in situ. In addition, this work emphasises the importance of the physical state of substrates in studies of the specificity and properties of lipolytic enzymes.

Splicing factors induce cystic fibrosis transmembrane regulator exon 9 skipping through a nonevolutionary conserved intronic element

In monosymptomatic forms of cystic fibrosis such as congenital bilateral absence of vas deferens, variations in the TG(m) and T(n) polymorphic repeats at the 3' end of intron 8 of the cystic fibrosis transmembrane regulator (CFTR) gene are associated with the alternative splicing of exon 9, which results in a nonfunctional CFTR protein. Using a minigene model system, we have previously shown a direct relationship between the TG(m)T(n) polymorphism and exon 9 splicing. We have now evaluated the role of splicing factors in the regulation of the alternative splicing of this exon. Serine-arginine-rich proteins and the heterogeneous nuclear ribonucleoprotein A1 induced exon skipping in the human gene but not in its mouse counterpart. The effect of these proteins on exon 9 exclusion was strictly dependent on the composition of the TG(m) and T(n) polymorphic repeats. The comparative and functional analysis of the human and mouse CFTR genes showed that a region of about 150 nucleotides, present only in the human intron 9, mediates the exon 9 splicing inhibition in association with exonic regulatory elements. This region, defined as the CFTR exon 9 intronic splicing silencer, is a target for serine-arginine-rich protein interactions. Thus, the nonevolutionary conserved CFTR exon 9 alternative splicing is modulated by the TG(m) and T(n) polymorphism at the 3' splice region, enhancer and silencer exonic elements, and the intronic splicing silencer in the proximal 5' intronic region. Tissue levels and individual variability of splicing factors would determine the penetrance of the TG(m)T(n) locus in monosymptomatic forms of cystic fibrosis.

Results with the Novacor assist system and evaluation of long-term assistance

OBJECTIVE

The great number of patients awaiting heart transplant and the shortage of donors has led to the increasing use of left ventricular assist devices (LVAD) for those patients that cannot wait only on medical therapy. In this study we analyze our experience in order to evaluate the possibility of long-term assistance.

METHODS

We have implanted LVAD Novacor in 36 patients with a mean age of 50.4 years. They were all critical candidates for transplant on high doses of inotrops. We evaluated the clinical and hemodynamic results and studied statistically the relative risk of complications at different time intervals of support.

RESULTS

In all cases we had a statistically significant improvement of: cardiac output, wedge pressure, pulmonary vascular resistance and mean pulmonary pressure. Eleven patients died on the device, 23 underwent heart transplant and two are still on the device. Causes of death were mostly related to cerebrovascular events or multiorgan failure. Seven of the 23 patients who underwent heart transplant died with a survival rate after transplant of 69.5% and an overall survival rate of 50%. Complications occurred in 33 patients with: 24 strokes, eight TIAs, four cerebral hemorrhages, three peripheral embolisms, seven cable infections, two pocket infections, two sepsis, two major lung infections, one mediastinitis, one right ventricular failure and three multiorgan failure. Time-related analysis showed that these complications occurred mostly during the first 3 months of assistance and this is particularly true for cerebrovascular events. The incidence of infections remained constant during the follow-up period. With a mean time of assistance of 203.1 days we had only two cases of device malfunction at 662 and 1297 days.

CONCLUSIONS

LVAD Novacor has provided reliable mechanical performance and good hemodynamic improvement. Most complications seem to occur in the first 90 days, therefore long-term assistance could be considered. A reduction of the high rate of thromboembolic complications remains mandatory to improve the clinical results.

Amylin compared with calcitonin: competitive binding studies in rat brain and antinociceptive activity

Binding studies for rat amylin (AMY) and salmon calcitonin (sCT) were performed on rat membranes prepared from pons and medulla oblongata of rats. The aim was to see whether specific binding sites for AMY and/or for sCT present in these areas could be relevant to some of the biological activities of the two peptides. Binding sites specific for [125I]AMY are present in the pons-medulla of rat brain as AMY, but not sCT, was able to displace radiolabeled AMY binding with an IC50 = 3.7+/-0.5x10(-10) M. In contrast, binding of [125I]sCT was displaced by both sCT and AMY, although with different potencies, the IC50 for sCT being 1+/-0.1x10(-11) M, and for AMY, 1.8+/-0.08x10(-7) M. The functional significance of the presence of these binding sites was evaluated in two different nociceptive tests, hot-plate and tail-flick. In the tail-flick test neither AMY (5-10 microg/rat, i.c.v.) nor sCT (10 microg/rat i.c.v.) showed antinociceptive activity, whereas in the hot-plate test AMY (10 microg/rat, i.c.v.) significantly increased the response latencies as did sCT (250 ng/rat, i.c.v.). These results demonstrated that a 40-fold greater dose of AMY is necessary to produce a comparable antinociceptive effect to that exerted by sCT. These findings are in accordance with the low affinity of AMY for sCT binding sites in rat pons-medulla. It is therefore suggested that the central inhibitory activity of AMY on pain perception involves interaction with sCT receptors whereas the selective AMY binding sites subserve other (as yet unknown) functions.

Functional analysis of cis-acting elements regulating the alternative splicing of human CFTR exon 9

The rate of exon 9 exclusion from the cystic fibrosis transmembrane conductance regulator (CFTR) mRNA is associated with monosymptomatic forms of cystic fibrosis. Exon 9 alternative splicing is modulated by a polymorphic polythymidine tract within its 3' splice site. We have generated a minigene carrying human CFTR exon 9 with its flanking intronic sequences and set up an in vivo model to study the cis-acting DNA elements which modulate its splicing. Transfections into human cell lines showed that T5, but not T9 or T7 alleles, significantly increases the alternative splicing of exon 9. Moreover, we found that another polymorphic locus juxtaposed upstream of the T tract, and constituted by (TG)(n)repeats, can further modulate exon 9 skipping but only when activated by the T5 allele. Then, we extended our studies to the mouse CFTR exon 9 which does not show alternative splicing. Comparison of human and mouse introns 8 and 9 revealed a low homology between the two sequences and the absence of the human polymorphic loci within the mouse intron 3' splice site. We have tested a series of constructs where the whole human exon 9 with its flanking intronic sequences was replaced partially or completely by the murine counterpart. The transfections of these constructs in human and murine cell lines reveal that also sequences of the downstream intron 9 affect exon 9 definition and co-modulate, with the UG/U 3' splice site sequences, the extent of exon 9 skipping in CFTR mRNA.

The sensitivity of cardiac markers: an evidence-based approach

The aim of this study was to determine whether, using an evidence-based approach, the results of the papers found in the literature are valid and sufficiently scientifically rigorous to be used to definitely address the problem of cardiac marker sensitivity in detection of acute myocardial infarction. In particular, the diagnostic sensitivities of myoglobin, creatine kinase (CK)-MB isoenzyme, determined as mass concentration, CK-MB isoforms, and of the two cardiac troponins, troponin I and troponin T, were reviewed using a priori formulated inclusion/exclusion criteria for judging the eligibility of studies to be included in the analysis. A clear final message derived from this systematic analysis is the unacceptably poor diagnostic sensitivity of all evaluated markers at patient admission, with substantial failure rate to rule out myocardial infarction at this time. Myoglobin is at present the most sensitive of the markers studied for excluding early AMI with an optimum timing of sampling at patient presentation and approximately 4 h later. However, this marker cannot be used by itself as a proportion of patients admitted to the hospital with a late infarction could be missed. The early rate of rise of CK-MB mass and troponin T is similar. Maximum sensitivity of these two parameters is achieved by the analysis of a second sample 6 to 12 h after admission. Additional larger studies are needed to address the question which troponin shows earlier release after myocardial damage, and to clarify the role of CK-MB isoforms as a possible early marker of myocardial infarction.

Hexarelin, a growth hormone - releasing peptide, counteracts bone loss in gonadectomized male rats

The age-related decline in growth hormone (GH) secretion has been implicated in the pathogenesis of involutional bone loss. Whether restoration of GH secretion might be helpful in maintaining and/or improving bone mass during aging is still unsettled. The aim of the present study was to examine the effects of 30-day treatment with hexarelin (HEXA, 50 microg/kg subcutaneously b.i.d.), a highly effective GH-releasing compound, on bone metabolism and bone mineral density (BMD) in intact and osteopenic gonadectomized (GDX) mature male rats. Serum total alkaline phosphatase (ALP, bone formation marker) and bone resorption markers (lysylpyridinoline, LP and hydroxylysylpyridinoline, HP) were measured before and 7, 14 and 30 days after treatment. BMD was measured by dual-energy X-ray absorptiometry at lumbar vertebrae, femoral metaphysis and diaphysis before and at the end of the experiment. In intact rats, HEXA significantly (P<0.05) decreased LP (-36.3%) and HP (-22.8%) excretion at day 7, whereas it did not change serum ALP activity and BMDs. In GDX rats, HEXA completely prevented the significant (P<0. 01) increase in urinary excretion of both LP (+143.8%) and HP (+119. 4%), the early decrease in ALP activity (-26.5%) and the significant (P<0.05) decrease in BMDs in the femoral metaphysis (-7.9%) and lumbar vertebrae (-6.8%) caused by androgen deficiency. The bone-protective effects of HEXA could be attributed, at least in part, to its GH-releasing activity since chronic-treated rats maintained the GH response to an acute challenge with HEXA. The evidence that HEXA, unlike GH, inhibits bone resorption indicates that other mechanisms contribute to the bone sparing effect of HEXA.

Effect of long term simvastatin administration as an adjunct to ursodeoxycholic acid: evidence for a synergistic effect on biliary bile acid composition but not on serum lipids in humans

BACKGROUND

Stimulated bile acid synthesis preferentially utilises newly synthesised cholesterol, raising the possibility that combination of simvastatin (an inhibitor of cholesterol synthesis) with ursodeoxycholic acid (UDCA; a stimulator of bile acid synthesis) may result in reduced bile acid synthesis and greater enrichment of the pool with UDCA than that achieved with UDCA treatment alone.

AIMS

To investigate the effect of simvastatin and UDCA given alone and in combination on serum and biliary lipid and biliary bile acid composition.

METHODS

Eighteen patients with primary non-familial hypercholesterolaemia were studied during treatment with simvastatin 20 mg/day, UDCA 10 mg/kg/day, and a combination of the two drugs. Each regimen was given in random order for three months following a three month lead in period.

RESULTS

Simvastatin significantly reduced serum low density lipoprotein (LDL) cholesterol but biliary cholesterol concentration remained unchanged. Combination of the two drugs had no synergistic effect on serum cholesterol concentration, but significantly increased the proportion of UDCA in the bile acid pool from 35% during UDCA to 48% during combination treatment (p<0.04).

CONCLUSIONS

Results showed that: (1) simvastatin reduces serum LDL cholesterol but has no effect on biliary cholesterol concentration, supporting the concept that newly synthesised cholesterol is not the preferential source for biliary cholesterol; and (2) combination of simvastatin with UDCA has the predicted effect of enhancing the proportion of UDCA in the pool. This effect may be of benefit in the treatment of cholestatic liver diseases.

Regulation of fibronectin EDA exon alternative splicing: possible role of RNA secondary structure for enhancer display

The fibronectin primary transcript undergoes alternative splicing in three noncoordinated sites: the cassette-type EDA and EDB exons and the more complex IIICS region. We have shown previously that an 81-nucleotide region within the EDA exon is necessary for exon recognition and that this region contains at least two splicing-regulatory elements: a polypurinic enhancer (exonic splicing enhancer [ESE]) and a nearby silencer element (exonic splicing silencer [ESS]). Here, we have analyzed the function of both elements in different cell types. We have mapped the ESS to the nucleotide level, showing that a single base change is sufficient to abolish its function. Testing of the ESE and ESS elements in heterologous exons, individually or as part of the complete EDA regulatory region, showed that only the ESE element is active in different contexts. Functional studies coupled to secondary-structure enzymatic analysis of the EDA exon sequence variants suggest that the role of the ESS element may be exclusively to ensure the proper RNA conformation and raise the possibility that the display of the ESE element in a loop position may represent a significant feature of the exon splicing-regulatory region.

Effects of amylin and salmon calcitonin on beta-endorphin-induced growth hormone and prolactin secretion in the rat

In this study we examined the possible interplay of amylin (AMY) and salmon calcitonin (sCT) in the central control of growth hormone (GH) and prolactin (PRL) secretion in male rats. For this purpose we first compared effects of central intracerebroventricular (i.c.v.) admininstration of various doses of AMY (2.5-2,500 ng/rat) and sCT (2.2-220 ng/rat) on beta-endorphin (beta-END, 0.5 microg/rat)-induced GH and PRL secretion. AMY and sCT dose-dependently inhibited beta-END-induced GH secretion, whereas only sCT was able to inhibit beta-END-induced PRL secretion. To examine whether the GH inhibitory effect of AMY was due to the possible cross-reactivity of AMY and sCT on the same receptors in the CNS, we pretreated some rats with the AMY antagonist (AMY8-37, 2. 5 microg/rat, i.c.v.). AMY8-37 significantly enhanced the GH-stimulatory action of beta-END. AMY8-37, administered prior to AMY and sCT, significantly removed the inhibitory effect of both AMY and sCT on beta-END-induced GH release, suggesting that both peptides mediate their response on GH through a common receptor. In vitro competition binding studies on rat hypothalamic membranes have shown that both AMY and sCT compete with [125I]rAMY binding with half inhibition (IC50) values of 3.6 x 10(-11) and 1.6 x 10(-10) M, respectively. Binding of [125I]sCT was inhibited by sCT with an IC50 of 1.09 x 10(-10) M and to a lesser extent by AMY with an IC50 of 1. 3 x 10(-6) M. Thus it is possible that the two peptides recognize a common hypothalamic receptor but with different affinities (sCT > AMY). Overall these data indicate that AMY behaves as a mimic of sCT in the central control of GH secretion. The failure of AMY, at variance with sCT, to modify the PRL-releasing activity of beta-END indicates that different receptor subtypes for sCT are involved in the endocrine effects of sCT and only those mediating the modulatory action of GH respond to AMY.

Investigation on the mechanisms involved in the central protective effect of amylin on gastric ulcers in rats

1. The mechanisms involved in the protective effect of amylin (administered into the brain ventricle, i.c.v.) on gastric ulcers induced by the oral administration of ethanol 50% (EtOH, 2 ml/rat) or indomethacin (indomethacin, 20 mg kg(-1), at a dosing volume of 5 ml) were investigated in rats. 2. The possible involvement of endogenous nitric oxide (NO) in the beneficial effect of amylin against EtOH-induced ulcers was examined. The inhibitor of NO-synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 70 mg kg(-1), s.c.) was injected 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH after a further 30 min. Rats were sacrificed 1 h after EtOH. L-NAME completely removed the protective effect of amylin. 3. The interaction between amylin and gastric nonprotein sulfhydryl groups was studied. The rats were treated with N-ethyl-maleimide (NEM, 25 mg kg(-1), s.c.) 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH 30 min after or by indomethacin 5 min after amylin. Rats were sacrificed 1 h or 6 h respectively after EtOH or indomethacin. NEM counteracted the protective effect of amylin against EtOH-induced ulcers but not against those provoked by indomethacin. 4. To determine whether amylin was able to promote ulcer healing, the peptide was injected 5 min after EtOH or 1 h after indomethacin. In the case of EtOH, the beneficial effect of amylin was lost whereas it was still effective on indomethacin-induced ulcers. 5. The results indicate that: the mechanisms involved in the antiulcer effects of amylin are different in these two types of gastric lesions probably because of the different etiopathology of various types of ulcers. Endogenous NO and nonprotein sulfhydryl groups are involved in the mucosal protective effects of amylin on EtOH and not on indomethacin-induced ulcers. Furthermore the effectiveness of amylin against indomethacin-induced lesions when administered after the ulcerogenic process has started suggests that amylin is involved not only in the protection but also in the healing mechanisms in this type of ulcer.

New lysosomal acid lipase gene mutants explain the phenotype of Wolman disease and cholesteryl ester storage disease

Deficiency of lysosomal acid lipase (LAL) leads to either Wolman disease (WD) or the more benign cholesteryl ester storage disease (CESD). To identify the molecular basis of the different phenotypes we have characterised the LAL gene mutations in three new patients with LAL deficiency. A patient with WD was homozygote for a null allele Y303X. The other two patients, with CESD, presented either homozygosity for T267I or compound heterozygosity consisting of Q64R and an exon 8 donor splice site substitution (G-->A in position -1). The mutants T267I and Q64R and the previously reported L273S, G66V, and H274Y CESD substitutions, overexpressed in stable clones, were found to be fully glycosylated and show an enzymatic activity of 3-8% of that of normal LAL. On the other hand, the delta254-277 mutant protein derived from exon 8 skipping and the Y303X protein were totally inactive. By transient transfection of hybrid minigene constructs, the CESD G-->A (-1) substitution resulted in partial exon inclusion, thus allowing the production of a small amount of normal LAL mRNA and hence of a functional enzyme. In contrast, a G-->A substitution observed in WD at position + 1 of the same exon 8 donor site resulted in complete exon skipping and the sole production of an inactive delta254-277 protein. In conclusion, LAL genotypes determine the level of residual enzymatic activity, thus explaining the severity of the phenotype.

Cysteine residues in human lysosomal acid lipase are involved in selective cholesteryl esterase activity

Human lysosomal acid lipase (LAL) catalyses the deacylation of triacylglycerol and cholesteryl esters in the acidic lysosomal compartment. Treatment of LAL with the reducing agent dithiothreitol affected the triacylglycerol and cholesteryl esterase activities differentially, suggesting the involvement of cysteine residues in determining substrate specificity. To identify the residues involved, human LAL cDNA, under the control of the T7 promoter and tagged with a herpes simplex virus coding epitope, was specifically mutated in order to introduce single amino acid substitutions of each of the six cysteine residues of mature LAL. All Cys-227 mutants showed selectively decreased activity towards cholesteryl oleate, while preserving that towards trioleylglycerol. Substitutions of Cys-236, Cys-240 and Cys-244 affected catalysis towards the two substrates to a variable degree, depending on the side chain of the amino acid introduced. The replacement of Cys-41 or Cys-188 did not result in the preferential cleavage of either one of the two substrates. These data indicate that Cys-227, Cys-236, Cys-240 and Cys-244 play a crucial role in determining LAL substrate specificity. We propose that these cysteine residues are involved in the hydrolysis of cholesteryl ester by affecting selectively the access of this substrate to the catalytic active site. In addition, the fact that the catalytic activity is never completely abolished in cysteine mutants demonstrates that LAL is not a thiol enzyme.