×÷Õߣºmungli prakash, jeevan k shetty, sudeshna tripathy, pannuri vikram, manish verma   

¡¾ÕªÒª¡¿  objective: in the present study we evaluated the paraoxonase activity and protein thiols level in south indian population with newly diagnosed hyperlipidemia.methods: the study was conducted on 55 newly diagnosed hyperlipidemic patients and 57 healthy controls. serum paraoxonase activity and protein thiols were estimated by spectrophotometeric method and lipid profile by enzymatic kinetic assay method. results: serum paraoxonase activity, protein thiols and high density lipoprotein levels were low and total cholesterol, triglycerides and low density lipoprotein levels were high in patients with hyperlipidemia compared to healthy controls (p<0.01). serum paraoxonase activity correlated positively with protein thiols and high density lipoprotein (p<0.01).conclusion: decreased paraoxonase activity and protein thiols were found in patients with hyperlipidemia. this may indicate the susceptibility of this population to accelerated atherogenesis and protein oxidation.

¡¾¹Ø¼ü´Ê¡¿  paraoxonase, protein thiols, hyperlipidemia, high density lipoprotein.

introduction

    hyperlipidemia is highly prevalent in indian population and known to contribute towards increased mortality and morbidity related to cardiovascular and cerebrovascular disorders £Û1£Ý.  mackness et al showed reduced paraoxonase (pon) activity in patients with hyperlipidemia £Û2£Ý. human serum pon is a calcium dependent esterase that hydrolyzes organophosphates such as paraoxone, diazoxon, sarin, and soman and also arylesters such as phenyl acetate £Û3£Ý. serum pon is a high density lipoprotein (hdl) associated enzyme synthesized mainly in liver. although the natural substrates for pon are unknown, pon decreases low density lipoprotein oxidation by its peroxidase activity and by preventing homocysteinylation of apo b 100 £Û4£Ý.

    the biological role of hdl is attributed to the presence of pon associated with it £Û3£Ý. a previous study indicates active site of pon contains free thiol (ª²sh) group present in cysteineª²283. this free ª²sh group is believed to donate reducing equivalent to pon and there by its reducing property £Û5£Ý. however recent studies indicated some other component of hdl other than free ª²sh of cysteineª²283 in pon is responsible for the biological role of hdl £Û5£Ý. recently the natural substrate and biological role of pon was reported by jakubowski £Û6£Ý indicating the role of pon in hydrolysis of homocysteine thiolactone into homocysteine (homocysteine thiolactonase activity).

    liver plays a key role in the synthesis of serum pon, and its serum levels were decreased in chronic liver diseases £Û7£Ý. hepatocytes synthesize albumin, which is a major plasma protein. the ª²sh groups present on protein are considered as major antioxidants in vivo and most of them are present over albumin. the levels of proteinª²sh in the body indicate antioxidant status £Û8£Ý.

    in the current work, we studied the a) activity of serum pon and protein thiols in south indian population with newly diagnosed hyperlipidemia and b) the relationship between pon activity and protein thiols in newly diagnosed patients with hyperlipidemia. 

    materials and methods

    subjects

    the study was carried out on 55 newly diagnosed hyperlipidemics not associated with any systemic disease and 57 healthy controls. mean age and sex of patients was 55¡À10 years and 36 males/19 females, and that of controls was 55¡À8 years and 39 males/18 females, respectively. patients with fasting total cholesterol >5.2mmol/l (>200 mg/dl) and triglycerides >2.2 mmol/l (>200 mg/dl) were considered having hyperlipidemia. hyperlipidemics having associated diabetes mellitus, coronary artery disease and any other systemic diseases, and post menopausal females were excluded from the study. informed consent was taken from all subjects involved and the study was approved by institutional review board. newly diagnosed hyperlipidemic subjects were recruited from kasturba medical college hospital; who came for routine health check up. the pon activity, protein thiol levels and lipid profile levels were estimated at the time of diagnosis.

    under aseptic conditions blood samples (5 ml) were drawn into plain vacutainers from anteª²cubital veins of controls and cases. the collected blood was allowed to clot for 30 minutes, and then centrifuged at 2000 g for 15 minutes for clear separation of serum. all assays were performed immediately after serum was separated.

    reagents and methods

    special chemicals like paraoxone, 5, 5¡¯ dithioª²bis (2ª²nitrobenzoic acid) (dtnb), were obtained from sigma chemicals, st louis, mo, usa. all other reagents were of analytical grade.

    pon was estimated spectrophotometrically by the method described elsewhere with minimal modifications £Û9£Ý. briefly, the assay mixture consists of 500  ¦Ìl of 2.2 mm paraoxone substrate in 0.1m trisª²hcl buffer, ph 8.0 containing 2mm cacl2 and 50 ¦Ìl of fresh serum specimen. the absorbance was monitored at 405 nm at 25¡æ. one unit (iu) of pon activity is defined as 1 ¦Ìmol of pª²nitrophenol formed per min per litre at 25¡æ, and activity was expressed as u/l of serum.

    serum protein thiols were measured by a spectrophotometric method using 5 5' dithioª²bis (2ª²nitrobenzoic acid) (dtnb) £Û10£Ý. fasting lipid profile was estimated by enzymatic kinetic assay method using automated analyzer, hitachi model 912. total cholesterol estimation was done by cholesterol oxidase method; hdl cholesterol was estimated by same method after precipitating the ldl, vldl, and chylomicrons £Û11£Ý. triglycerides were estimated by enzymatic mixture containing lipoprotein lipase, glycerol kinase and glycerolª²3ª²phosphate oxidase and peroxidase £Û12£Ý. low density lipoprotein levels were calculated by using friedewald's formula.

    statistical analysis:

    the results were expressed as mean   standard deviation (sd). a p value of <0.05 was considered statistically significant. statistical analysis was performed using the statistical package for social sciences (spssª²10, chicago, usa). independent samples t test was used to compare mean values. pearson correlation was applied to correlate between the parameters.  table 1  serum paraoxonase and lipid profile parameters in healthy controls andhyperlipidemia cases, values expressed in mean¡Àsd. *p<0.01 compared to healthy controls.

    results

    as depicted in table 1, hdlª²cholesterol, pon activity and protein thiol levels were significantly decreased, and total cholesterol, triglycerides and ldlª²cholesterol were increased in cases compared to healthy controls (p<0.01). on applying the pearson correlation, serum pon activity correlated positively with protein thiols and hdlª²cholesterol (p<0.01), and pon activity correlated negatively with total cholesterol (p<0.01), triglycerides (p<0.05) and ldlª²cholesterol (p< 0.01). 

    discussion

    in our study we found significant decrease in pon activity in hyperlipidemia patients. this decrease in pon activity may be associated with decrease in levels of hdlª²cholesterol. decrease in hdl associated pon activity and increase in ldlª²cholesterol in hyperlipidemia may be favorable for atherogenesis process, thus predisposing patients with hyperlipidemia for premature coronary artery disease. the exact mechanism of antiª²atherogenic function of hdl and its associated components is not clear at present but the role of hdl associated pon activity in this process is increasingly stressed in recent times £Û13£Ý.

    although several studies proposed the antiª²oxidative and antiª²atherogenic nature of pon but the exact mechanism is still not clear. recently the natural substrate and biological role of pon was reported by jakubowski £Û6£Ý. their study indicates role of pon in hydrolysis of homocysteine thiolactone into homocysteine (homocysteine thiolactonase activity). homocysteine thiolactone is unstable compound and can bind to proteins at lysine residues. this nª²homocysteinylation of proteins alters protein's structure and increases its susceptibility to proteolysis. nª²homocysteinylation of pon (or other component of hdl regulating its activity like apolipoprotein a1) decreases its activity. this, decrease in pon activity may initiate a positive feedback mechanism, since reduced pon activity will cause further accumulation of homocysteine thiolactone and may augment protein homocyteinylation £Û4, 6£Ý.

    in our study, we found significantly decreased levels of protein thiols in hyperlipidemia patients and they correlated positively pon activity. albumin is major plasma protein and contributes greatly to plasma proteinª²sh pool £Û14£Ý.  homocysteinylation of albumin at cysteineª²34 may make it increasingly susceptible to oxidative degradation £Û6£Ý. increased homocysteinylation may also be responsible for decreased pon activity in patients with hyperlipidemia. since pon activity is responsible for hydrolyzing homocysteine thiolactone into homocysteine, decreased activity of pon may be the possible cause for increased protein homocysteinylation and oxidative degradation of plasma proteins leading to decrease in available sh groups £Û4£Ý. this may explain the possible cause for positive correlation between protein sh and pon activity in hyperlipidemia patients.

    mungli prakash, et al. serum paraoxonase activity and protein thiols in patients with hyperlipidemia    in conclusion, decrease in hdl associated paraoxonase activity and major plasma antioxidant, protein thiols, in patients with hyperlipidemia may increase accelerated ldl oxidation leading to atherogenesis and protein oxidation. 

¡¾²Î¿¼ÎÄÏס¿
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