Apolipoproteins: Significance and measurement

ABSTRACTS OF A N N U A L MEETING

1980 38 1

SHOULD WE MEASURE HDL CHOLESTEROL?

LEONA. S I M ~ U SLiiiid Lahorotori.. Sr I'iriwnr '.c tlo.c.,itol, .Sj.c/nr,v LDL and HDL appear to have opposing roles in plasma lipid transport, LDL traiisporting cholesterol i'i-om the liver 10 peripheral tissues and HDL transporting in thc reverse direction. This may explain in part the positive iissociiition between LDL and atherosclerosis development, and the negative association between HDL choleslerol and atherosclerosis. I.C. a protective role. Numerous epidemiological studies have confirmed that patients with vawilar disease (coronary artery disease, cerebrovascular disease or peripheral vascular disease) have significantly lower HDL cholesterol concentrations than matched controls. The variations in HDL cholesterol concentration which account for important changes in vascular disease risk are of the order of 0.10-0.13 inmol 1. This degree 01' precision is within the reach of chemical analysers currently in use. after extra attention to choicc of incthod and quality control. Unfortunatcly. the epidemiological data base draws upon elderly populations ( > 50 yi-) mostly resident in North America. Although it is likely that HDL cholesterol also operates as a protective factor in 1 ounger people, an unacccptable degree ofextrapolation may be required for the interpretation ofabsolute HDL cholesterol concentrations in Australian subjects. We are aware of many factors that inllueiice HDL concentrations, e.g. physical fitness, cigarette smoking. alcohol intake, obesity, vegetarianism and so on. However, we do not know whether it is beneficial to elevate an apparently 'low' HDL cholesterol Icvcl. Because of problems with accuracy and validity. routine estimation of HDL cholesterol concentration, as part of a n ovcrall lipid profile, docs not secm indicated in the present state of the art. Houevcr. 21 policy of selective measurement is fully justified. e.g. in assessing unusual lipid transport disorders, in I~llowingncwcr therapies, or in basic research. MEASUREMENT OF SERUM TOTAL AND HIGH DENSITY LIPOPROTEIN CHOLESTEROL

.\ .Mi,Ilwi~rii? NKHOLAS D. H. BALALS Dc>pur.tnii.rr/of C'lzc'rnicul pa tho log^, Prinw H w i r ~ ~ 'Ho.vpitti1. More than L)O",>of Australian laboratories enrolled in the RCPA AACB Lipid Quality Control programme use enzymatic reagents to measure cholesterol. Usingcriteria similar to those ofthe Center for Disease Control. U.S.A.. accuracy is improving from previous levels -approximately 70",, of laboratories are now achieving acceptable recult\ By carefully evaluating the differing kinetic behaviour of commercially available cholesterol reagents (kits) wlth both serum and 'aqueous' (detergent solubilised) cholesterol standards, it is possible to achieve good agreement with Abell-Kcndall reference values with somc reagents. The use of Matrix Standards~lyophiliiedor frozen serum accurately assayed by the Abell-Kendall referencc method extends the accuracy further and is recommended for multi-point calibration of total cholesterol assays. Data will be presented to justify this approach. A prime task for the Australian Lipid Standardization Programme is to make readily available biological matrix reference inaterials accurately calibrated for cholesterol concentration. The nccurate measurement of HDL Cholesterol (HDLC) has been hampered by confusion regarding the optiiniil separation technique to be used. Based on an examination of practicability and accuracy cinteria. Polyethylene Glycol 6000 ( 100 g;l final concentration) is the recommended scpiimtioii incthod followed by cnz>miitic measurement of HDLC in the supernatant. Precautions regarding sample collection and storage will be discussed. Analytis of HDLC should be regarded as a separate assay using increased sample reagent ratios (Min 1-40)and multiple low-lcvcl standards. Data will be presciitcd to justify these recommendations and a schcmc proposed to monitor variability of both the separation and analysis components of the assay. Imprecision of HDLC' assays should not exceed & lo",, (2 SD) at a lcvcl of 1.0 mmol I if the assay is to be clinically useful. APOLIPOPROTEINS: SIGNIFICANCE AND MEASUREMENT

Dc~iil,ur/n?ciitof Clini(.~IBioc,heniistr>,,Flindtw M r ~ d i i ~ c rCcntw. l Ar/c~/trith Plasma lipoproteins arc particles containing lipid (cholesterol, cholesteryl ester. triglyceride, phospholipid) and specific proteins (apoproteins). Lipoproteins are generally classified by density. This is a useful operational classification and also onc which bears some relationship to function. i n that the less dense lipoproteins (chylomicrons), very low density lipoprotein (VLDL). low density lipoprotein (LDL) transport lipid from intestine or liver. whcreas high density lipoproteins (HDL), it is thought, play a part in the passage of cholesterol from extrahepatic cells to the liver. The apoproteins serve a number of functions, but the function of many is unknown. Known functions include: 21. Structural functions. ApoB is an essential protein for the formation ofchylomicrons. VLDL and LDL. ApoA apprars essential for HDL formation

G. D. CALVtKT

382

R O Y A I ( ' O L l . l : < i I . Ot P A I'lIOl~OGIS1'S0 1 , AUSl IIAI.ASIA

I'aihology (19x1). 13. April

ApoC'll is 'I coeiiLgiiic for lipoprotein lipase. and apoAl for lecitliiii:cliolestrrol ac) itr;illsii.I.Ll\e I X ~ A T . c. Markci-s. alltn+iiieuptake and cataholisiii ol'\pecific lipoproteins. ApoB i n L D L I\ taken u p at reccptoi sites on cxti-ahepciticcell\. Apot: ins! act as l?cd Thew pow technical problem\ peculiar t o Iipo ;I Antigen c x p o ~ t i i - cAn . antibod! rai\ed against ;I delipidated apoprotcin ma! not fully recognise the apoprotein wlicii i t l'orms pai-t 01 ;I Iipoprotcin. Portions ol'the apoprotcin inah he masked by lipid. I t inaq be nccessai-y t o ilelipidatc. 01-in \oiiie other uaq break up the lipoprotein beliire asbay. b. T h e inobilit) o t i i c o m p l e x lipoprotein in a n eleclroitiiinuno~i\s~iq ma\; be t c d by the site o f t h e particle uid bq t h e other protcin component\ o l ' t h e lipoprotein. I n iiii electroimmuno, jtandnrds should be siiiiiliir and 111 il \imllar llliltrlx to the tc \t \olulion. I t maq be \houri i i i the future that ~ i b \ ; i y s lor ( s a q ) plasina ;ipoB. apoAI and apoE will ;illow iiioi-c accuralc pi-ediction 01' the ri\k 0 1 ' de\cloping ischaeiiiic tieai-t di in the individual than tlic arsays nou aiailahle f o r cIiolc\tci-oI. triglqcei-ide a n d tlnL-ctiolcstri-ol. but this has not yet been demonstrated h. ('ocii/qine 'ictibit;

PREDISPOSING FACTORS TO ARTERIAL THROMBOEMBOLISM: CAN WE DETECT THOSE AT RISK?

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popii Iii Ii o i i \ ii t r i\ k o I' ii t hei-oscIcro. iiid t hroin b m i \ arc k nou n t o include environ m e n t ;iI. diet;ir) . inetaboliu a r i d tli\e;i\e a\\ociatioii\ that maq be altered (31. innnipul,ited in individual patient\. Les\ is known or cat~ihli\hc.tlabout the importance of changes in platelet 01- coagulation hehaviour that m;iy contribute to ongoing \;i\ctiI:ir disea\e oi. to i i c t i ~ occlusice episode\. Current research tends to suggest a n important role foi- platelets i n t h e gciie\i\ o f atheroina. e\pecially \iiice the discovery 01' platelel growth factor which ma? have mutageiiic propertics on vc\\el smooth i n u j c l e cell\. Addition;il roles are suspected for platelet and vessel prostaglandins and current w o r k is dc\oted t o di\cocering h o w these important incdiatoi-5 and inhibitors of platelet activity m a y contribute to ttii-ombo\i\ C'oiigulLiiioii a i i d platelet Iiiiictioii \tiidics have been pel-i'orined in patients i ~ i t hartei-ial thrombotic diseaac. M a n ) oi thesc haLc been iii\cstig;ited a t the t i m e of presentation. Global test\ of coagulation and fihrinolysis have n o t \hour1 an> chiiiige. hut ii \igniiicant enhiincement ofthe threshold response to Adenosine Diphosphate has hcen i'ound a n d man) oi' thew patient5 had enhanced release of platelet /I thrornboglobulin. Radioirniiiuiioass~i)of rhronibozaiic Hz \ h o \ \ e d increased j>nthesi\ alter collagen stimulation in many patients with ongoing thrombosis. The\c i-e\ult\ \ugpe\t th;it eiihaiiccd thromboxane synthesis and increased i-cspoiisiveness t o ADP are important ,t\\ocialioii\ 01' ttii-oinbo\i\. I t remains to be drtcriiiined whether these changes have predictive valuc.

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PREDISPOSING FACTORS TO VENOUS THROMBOEMBOLISM: CAN WE DETECT THOSE AT RISK?

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01' coagulation o r platelet liinction caii reliablq. selectively and sensitively predict the risk of venous (1ii-oiiibociiibi)Iisiiii n tlie inajorit) 01' paticiits. even in the high risk groups. O n this basis rouhne. extensive. prcoperarite blood te\ting ciiiinot be recominended, hut 2 relatively simple tests can be justified. Polycythaeinia c o n s t i t u t e \ iiii increased risk siluation hence doubling the value ofa pre-operative haeinoglohin estimation. Partial rlii-oiiibopl;i\tiii t i m c (I'TTK) is more commonly rccoinniended a s ii screening test for iiii unsuspected bleeding tendency. hut con\iderable e l tdeiice ha\ accriicd recentlq t o suggest that a significant shortening <)I' the PTTK is a\\ociated \citIi ;in incrca\cd risk of\enou\ thi-oinhoembolisin. Shortening o f the PTTK is inost coiiimonlq seen i n tr~uinati7edpatient\ o r p a t ie n t \ w i t h renal or inalignant disease. Risk asiessincnt o f venous throinhoemboli\in is therehi-c inore depciideiit on clinicnl a\sessiiient and ii knowledge o f tlic known predisposing factors. Both the iiidi\idual p a t i e n t s and tlie particulai- procedure to be pcri'ormed need to be assessed in this light. A full personal and L i i i i i l ~histor! I \ e>\ential ii pi-evious e p i w d e o f b e n o t i j thrombosis represents a highlq significant risk lietor. I i i d i L d u a l \ u i t h unexplained recurrent t h r o i ~ i b ~ ) c i i i b ~ ~ o l irs ai i signiiicaiit i family history \hould he tehted lor rare abnormalities \uch a s congenital antithrombin I l l deficiency or ;I d)sfibrinofciiaeini~~.Particulni- situations wliicli c;iu\c a n acquiicd antittirombin Ill deficiency \Late such 215 recent heparin therapy iiiid liver disease. D.I.C.. te\t