J Clin Pathol 1983;36:938-947

Angiotensin-converting enzyme and its clinicalsignificance a review

PETER R STUDDY*, RUTH LAPWORTH, ROGER BIRD

From the Royal Northern Hospital, Holloway Road, London N7

SUMMARY There have been considerable advances in understanding the metabolic role of theendothelial lining cells of the blood vessels. Angiotensin-converting enzyme activity is concentratedin these cells, especially those lining the pulmonary circulation. The enzyme exerts control over

systemic vascular tone indirectly through the powerful pressor effect of angiotensin II. A number oftherapeutic agents are now available which directly inhibit converting enzyme activity and therebyeffect a reduction in blood pressure.Macrophages are the source of increased angiotensin-converting enzyme activity commonly

found in association with active sarcoidosis. A better understanding of this phenomenon may givefresh insight into this puzzling condition. Pulmonary endothelial metabolism is affected by lunginjury and it is likely that in this situation changing activities of serum angiotensin convertingenzyme may indicate the extent ofdamage and the response to therapy. The full clinical significanceof serum ACE measurements has yet to be established. However, raised activities have beenreported in a number of other conditions and diabetes mellitus and hyperthyroidism are ofparticular current interest.The numerous methods and reference ranges described in the literature for the measurement of

serum ACE activity require further assessment, and there is a clear need for an accepted referencemethod.

Angiotensin I, a decapeptide generated by action ofthe enzyme renin on a glycoprotein substrate angio-tensinogen, is converted to the pressor octapeptideangiotensin II. The exopeptidase responsible for thisconversion was first identified and isolated in plasmaby Skeggs et al,'2 who named it angiotensin-converting enzyme. This enzyme is a halide-activated, EDTA-sensitive, peptidase that catalysesthe cleavage of dipeptidyl residues from the COOHtermini of peptide substrates,3 and releases His-Leufrom the COOH terminus of the decapeptide angio-tensin I.The ready conversion of angiotensin I to angio-

tensin II was assumed to take place in the circulationuntil Ng and Vane4 recognised that the enzymeactivity present in plasma was insufficient to accountfor the rapidity of "in vivo" conversion, and demon-strated most conversion of circulating angiotensin I toangiotensin II occurred during passage through thelungs. Stanley and Biron' confirmed the importance

*Present address: Harefield Hospital, Harefield, Middlesex...Accepted for publication 17 January 1983

of lung as a site of angiotensin-converting enzymeactivity by demonstrating reduced angiotensin Iconversion in dogs on cardiopulmonary bypass; thelowered rate of conversion, however, was still higherthan that in blood alone.The pulmonary vascular bed is uniquely placed to

modify levels of circulating vasoactive substances,being immediately up stream of the high pressuresystemic circulation and receiving virtually the wholecardiac output. However, the fact that blood pressurecontrol is maintained, albeit at a lower level, duringcardiopulmonary bypass indicates that angiotensin-converting enzyme is present in vascular beds otherthan the lungs.6 Cushman and Cheung7 measuredangiotensin-converting enzyme activity in a largenumber of rat tissue homogenates and found highspecific activities in lung and in segments of thedigestive tract but the highest activities in matureanimals were in the testes and epididymis associatedwith tubular fluids, but not in sperm cells.

Other sites in which conversion of angiotensin I toII has been reported to occur include the renal vascu-lature of humans8 and dogs,9-" the vascular bed ofthe intestine,'2 the hind limb of the dog" forearm

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Angiotensin-converting enzyme and its clinical significance - a review

arteries and hand veins in man.'3 In 1977 Lanzillo'4isolated angiotensin-converting enzyme from humanserum and as expected, found high concentrations inthe vascular bed of the lungs.

Electron microscope studies with microperoxidaselabelled antibody against angiotensin-convertingenzyme localise the enzyme activity to the luminalsurface of the capillary endothelium and associatedcaveolae in the aorta, lungs, and in other vascularbeds'5 and this work provided visual confirmation ofthe many reports linking angiotensin-convertingenzyme to the vascular endothelium. 1614Therefore,angiotensin-converting enzyme is a membrane-bound glycoprotein located predominantly in endo-thelial pinocytic vesicles. Circulating angiotensin-converting enzyme almost certainly derives fromsloughing of, or secretion from, the endothelium.The unique feature of the pulmonary circulation interms of the renin-angiotensin system is that it is theonly vascular bed through which angiotensin II passeswithout inactivation.'7 Ferreira and Vane'8 investi-gated bradykinin, another vasoactive substance, andfound its activity disappeared during circulationthrough the lungs, leading Ng and Vane'9 to suggestthat the same pulmonary enzyme responsible forconversion of angiotensin I may also inactivate brady-kinin. Supporting evidence for this hypothesisincludes finding that bradykinin is a competitiveinhibitor of angiotensin I conversion and that brady-kinin potentiating factors (peptides from the venomof a Brazilian pit-viper Bothrops jararaca) not onlyinhibit the destruction of bradykinin by plasma, butalso inhibit the conversion of angiotensin I by lungextracts.20 The isolation of snake venom inhibitors(Bothrops jararaca) of angiotensin-convertingenzyme4 enabled studies to be made showing theimportance of the renin-angiotensin system in themaintenance of blood pressure. A number of otherinhibitors have been characterised and synthe-sised2-24 and shown to possess similar pharmaco-logical properties with inhibition of the vasopressorresponse to angiotensin and enhancement of the vasodepressor effect of bradykinin.22 It was later es-tablished that pure converting enzyme from rabbit,25and pig lung26 catalysed sequential release ofPhe-Argand Ser-Pro from the COOH-terminus of bradykinin.The details of the two protease pathways that share

angiotensin-converting enzyme are summarised inFig. 1. The one enzyme acting upon its two currentlyrecognised substrates has been classified as a pepti-dyldipeptide hydrolase and named "peptidyl dipep-tidase" by the IUPAC-IUB Commission on Bio-chemical nomenclature. Since large amounts ofangiotensin I converting enzyme are found in plasmaand in a number of organs and since angiotensin Iinfused in supraphysiological concentration is rapidly

converted into angiotensin II in vivo, the conversionstep has not generally been regarded as rate-limitingfor angiotensin II production. Relatively little atten-tion has been given to the regulation of convertingenzyme activity in vivo.However, several reports have suggested that

alterations in angiotensin I conversion occur in pul-monary disease or when abnormalities in pulmonaryfunction are experimentally induced. Manipulationof pulmonary vascular surface area and mean transittime through the pulmonary capillary bed, bychanging pulmonary venous pressure, altered con-version of infused angiotensin I in the isolatedperfused dog lung.2" Molteni28 studied changes inboth serum and pulmonary angiotensin-convertingenzyme activity and renal renin content in miceexposed to hypoxia in a hypobaric chamber at 0 5atmospheres. A small fall in the activity of bothenzymes was demonstrated at two days followed by aprogressive rise reaching significance (p < 0-005) bynine days. The finding in dogs that acute hypoxiainhibited the activity of angiotensin-convertingenzyme suggested a possible explanation for the lowplasma aldosterone and raised plasma renin activityfound at altitude.29 Milledge and Catley30 studied theeffect of moderate exercise continued for 120 minupon renin, aldosterone and angiotensin-convertingenzyme. After 60 min exercise while breathing roomair plasma renin activity and plasma aldosterone rosewhile angiotensin-converting enzyme activity re-mained unchanged. The same subjects continuedexercise for another 60 min while breathing 12- 8%oxygen and further blood samples showed a 30% fallin angiotensin-converting enzyme activity and a fur-ther rise in renin and aldosterone. These findings mayhave implications for humans who suffer from moun-tain sickness on exercise at altitude. Therefore,alterations in pulmonary function may have a regula-tory effect on the renin-angiotensin system throughthe activity of angiotensin I converting enzyme.

This led to studies of the enzyme's activity inpatients with pulmonary disease.3' Depressed con-verting enzyme activity has been reported in patientswith several kinds of pulmonary disease, includingchronic bronchitis and emphysema, carcinoma of thebronchus, bronchial asthma and cystic fibrosis.3'-33This raised the possibility that the reduction in con-verting enzyme activity may relate to a loss of vascularendothelial surface area; but in fact no correlationwas demonstrated between the severity of the pulmo-nary disease and reduction in converting enzymeactivity.

In contrast, Lieberman in 197532 demonstrated thatserum converting enzyme activity was raised insarcoidosis and his finding has been confirmed bymany, and is discussed further.

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Prekallikrein Angiotensinogen4 Kininogen Plasma

Kallikrein 11 ,reninBradykinin Angiotensin I

PIasmQ Pulmonary Ikinin'se converting Fig 1 Diagram oftwo protease pathways that share

Bradykinin Angiotensin II angiotensin-converting enzyme.degradationproducts

Aminopeptidose Tissueangotensinases

Angiotensin III Angiotensindegradationproducts

Properties of anglotensin-converting enzyme

Human angiotensin-converting enzyme (ACE) hasbeen successfully purified by a combination of chro-matographic and electrophoretic techniques. It is anacidic glycoprotein (iso-electric point 4- 6) consistingof a single polypeptide chain of molecular weightabout 140 000.14 The activity of the enzyme dependsupon the presence of chloride ions, while chelatingagents, sulphydryl compounds, heavy metals andcertain peptides are inhibitory. The presence of zinchas been demonstrated in rabbit and canine en-

zymes343 and is thought to be closely associated withhuman angiotensin-converting enzyme. In phospho-saline buffer, serum ACE is most stable at pH 8-0 -

8- 8 with an optimum pH of 8- 3.36 37 The most exten-sively characterised of the pure enzyme preparationsis an extract from rabbit pulmonary membranes sepa-rated with DEAE-cellulose ion exchange, SephadexG-200 gel, and lectin affinity chromatography.'22'Apart from variations in the glycoprotein content thehuman enzyme appears to be very similar if notidentical to rabbit and canine enzymes.There does not appear to be any significant differ-

ence in behaviour between raised serum angiotensin-converting enzyme from sarcoid patients and serum

angiotensin-converting enzyme from normal sub-jects, with respect to temperature and pH properties.At the present time isoenzyme forms have not beenidentified.37 Serum samples may be stored prior toassay, for we have found that the enzyme's activity isstable for at least 20 days at 25°C, 1 month at 4°C and6 months at -20°C.

Methods of measuring serum anginsn-converngenzyme activity

A number of different methods for determiningangiotensin-converting enzyme (ACE) are available.

Cushman and Cheung38 described a method for as-saying the enzyme's activity in rabbit lung extractsand measured the activity in terms of the rate ofrelease of hippurate from a substrate analoguehippuryl-L-histidyl-L-leucine, which substitutes forangiotensin I. Cushman and Cheung's method asmodified by Lieberman32 is simple to perform and isnow widely used to measure serum ACE activity. Thetest serum and substrate are incubated under con-trolled conditions. The reaction is terminated byacidification, and after separating the unhydrolysedsubstrate by extraction with ethyl acetate, hippurateis measured spectrophotometrically by determiningits absorbance at 228 nm. In the authors' experiencethe method is relatively insensitive and difficult tocontrol when used for the routine assay of serumACE. All traces of ethyl acetate must be removed byevaporation for this substance absorbs strongly at 228nm and therefore interferes with the assay. Lipaemicor haemolysed serum samples may cause interferencein the extraction step. Chiknas39 described a modifi-cation of Cushman and Cheung's method38 in whichhigh pressure liquid chromatography is used tomeasure the hippuric acid end product. This pro-cedure overcomes a number of problems encoun-tered with the spectrophotometric assay.An alternative spectrophotometric method444'

uses cyanuric chloride/dioxan as reagent which reactswith liberated hippurate in the presence of phosphatebuffer to yield a chromogen that is quantified by itsabsorbance at 382 nm. Recently, a two-point kineticassay based on this reaction has been described,42which is said to be unaffected by jaundiced orlipaemic specimens.

Friedland and Silverstein43 described a sensitiveand reproducible fluorimetric assay using hippuryl-L-histidyl-L-leucine as substrate in which the rate ofproduction of L-histidyl-L-leucine is quantifiedspectrofluorometrically by the formation of a fluores-

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cent adduct with o-phthaldialdehyde. This method ishighly sensitive, reproducible and unaffected byhaemolysed, jaundiced or lipaemic sera. An inherentcriticism in measuring the production of L-histidyl-L-leucine is that L-histidyl-L-leucine itself is susceptibleto hydrolysis by serum peptidase enzymes. However,under the conditions of the assay, hydrolysis of L-histidyl-L-leucine is minimal and no correction needbe made. In a similar assay Friedland and Silverstein4'used the physiological substrate angiotensin I insteadof the synthetic analogue. This is converted by serumACE to angiotensin II and L-histidyl-L-leucine andthe latter substance was quantified fluorimetrically asin the previous method. There was a highly significantcorrelation between measured serum ACE activitydetermined with angiotensin I compared with theanalogue substrate hippuryl-L-histidyl-L-leucine,(r = 0 903).

In an alternative method described by Dorer,45hippurylglycylglycine is used as the substrate andserum ACE activity measured in terms of release ofglycylglycine. The glycylglycine is quantified by a

ninhydrin reaction. Glycylglycine is less susceptibleto hydrolysis by peptidases in serum than histidyl-L-leucine but the method is complicated by the need forprior dialysis of serum. Other fluorogenic sub-strates4647 have been used.A rapid two-step colorimetric assay has recently

been described by Kasahara and Ashihara,4 basedupon the formation of the quinoneimine dye formedfrom the substrate p-hydroxyhippuryl-L-histidyl-L-leucine, and the results are said to correlate closelywith Cushman and Cheung's method (r = 0 98). Thismethod is to be made available as a test kit (ACEcolor, Fujizoki Pharmaceutical Co Ltd, Tokyo,Japan). (Agent: Diamed Diagnostics LTD. England.)

Radioassays, using radiolabelled substrates haverecently been developed for the determination ofserum angiotensin-converting enzyme activity. Themethod described by Rohrbach49 used (Glycine-1-'4C) hippuryl-histidyl-leucine as a substrate, where-as the method described by Ryan etal5l now commer-cially available (Ventrex Laboratories Inc, Portland,Miami), uses (P- (3H) benzoyl) hippuryl-glycyl-glycine. Both methods measure enzyme activity interms of release of radiolabelled hippuric acid fromthe substrate, and in both methods haemolysed andlipaemic samples can be processed. As in the methodof Cushman and Cheung, hippurate is separated fromunhydrolysed substrate by extraction with ethylacetate. However, ethyl acetate need not be removedby evaporation as the hippurate content of the ethylacetate extract can be quantified directly by liquid-scintillation counting, thus eliminating a potentialsource of error. Ryan compared their assay with thatof Cushman and Cheung (as modified by Lieberman)

and obtained a highly significant linear correlation (r= 0-891). McGregor, Roulston and Bird (unpub-lished data) compared the assay of Friedland andSilverstein with that of Ryan et al and also obtained ahighly significant linear correlation (r = 0 98).

Clinical applications ofserum ACE measurements

SARCOIDOSIS

IntroductionSarcoidosis is an enigmatic multisystem granulo-matous disorder of unknown aetiology.5" " Theclinical features and chest radiograph appearance areoften very characteristic but confident diagnosisrequires histological confirmation. Problems inmanaging sarcoidosis include diagnosis in atypicalcases, for in a proportion of patients sarcoidosis isunsuspected until sarcoid granulomas are found atbiopsy, and the assessment of activity in chronicdisease.A number of biochemical abnormalities are

common in sarcoidosis, including hypercalcaemiaand hypercalcuria,52 hydroxyprolinuria," hyper-globulinaemia,54 changes in individual immuno-globulin concentrations,55 increased levels of lyso-zyme,56 collagenase or elastin;57 S but none of theseabnormalities is specific or sensitive enough todiscriminate sarcoidosis from a number of otherconditions.

Angiotensin-converting enzyme (ACE) activity isfrequently raised in the serum and virtually always ingranulomatous lymph node tissue of patients withsarcoidosis.59-6' This association between serumACE and sarcoidosis was reported by Lieberman.62who suggested serum ACE may be a useful diagnostictest and that serial measurements of activity may helpin clinical management.

Lieberman's observation has been confirmed bymany,37 43 49 63-79 All demonstrated significantlyraised serum ACE activity in sarcoidosis, and mostshowed falling enzyme activities with corticosteroidsor spontaneous remission. Rising activities wereassociated with relapsing sarcoidosis.

Origin ofincreased serum ACE in sarcoidosisThe origin of the increased ACE activity in sarcoid-osis is debated, but is thought to reflect stimulation ofthe monocyte macrophage system. The high turnoversarcoid granulomas are sites of considerable macro-phage activity and receive massive reinforcements ofmonocytes to sustain their activity.80 The epithelioidcells are derived from macrophages, appear bio-synthetically active8' and are the most likely source ofthe increased serum and tissue ACE activity as well as

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other biologically active molecules, including lyso-zyme, glucuronidase and elastase. This hypothesis issupported by the following evidence. Human andrabbit monocytes, the precursors of macrophages,can be induced on culture to secrete ACE in con-centrations up to 700 times that obtained beforeinduction.8283 Immunofluorescence studies localiseangiotensin II specified fluorescence to epithelioidcells, indirectly locating abundant ACE activity84 tospecific cells in sarcoid granulomas.

Diagnostic sensitivityThe reported diagnostic sensitivity of serum ACEvaries; but overall some two-thirds of patients withactive sarcoidosis have raised enzyme activity. Aneven higher proportion will show increased ACEactivity in fluid aspirated from the lungs at broncho-alveolar lavage and virtually all will show increasedACE activity in biopsied granulomatous lymphnodes."59-6Serum ACE activity is independent ofsex, race and

diurnal variation in normal adult subjects (> 18 y ofage) and shows no significant fluctuation throughoutadult life. In normal healthy infants, children andearly adolescents serum ACE activity is substantiallyhigher and more variable than in adults32 76 77 85 andthen declines to adult activities as growth ceases.Because of this variability, a normal range of serumACE activity for patients under 18 y remains to begenerally established.

67In our series, serum ACE activity was measuredin normal adult subjects (> 18 y), in adult patientswith sarcoidosis and other granulomatous or res-piratory diseases using the spectrofluorimetricmethod of Friedland and Silverstein. The normalrange was from 16 to 52 nmol/min/ml with a mean of34 nmol/min/ml (mean + 2SD = 34 ± 18), obtainedfrom 80 age and sex matched healthy adult subjects,(mean age 42 y, range 18-60 y).Compared to normal subjects serum ACE activity

in 146 sarcoid patients was significantly raised (p <0.001) with a mean activity of 54 + 27 nmol/min/ml(mean + SD), (Fig. 2). Serum ACE was raised inabout half with active untreated sarcoidosis and inone-fifth after corticosteroid treatment. The highestindividual enzyme activities are found in thosepatients with extensive pulmonary and hepatic in-volvement. Serum ACE activity appears to be mostinfluenced by the extent of sarcoidosis, for most in-vestigators demonstrate a clear trend for activity toincrease in parallel with the degree of radiographic orclinical abnormality. In our series this trend was veryapparent, for raised serum ACE was found in 27% ofsarcoidosis patients with a normal chest radiograph,in 56% with hilar gland enlargement but clear lungfields (stage 1) in 72% with pulmonary infiltration in

Studdy, Lapworth, Bird

association with hilar gland enlargement (stage 2) andin 80% with active sarcoidosis and pulmonary infil-tration alone (stage 3).

Serum ACE in other granulomatous conditionsSerum ACE activity was increased in 6% of a groupof patients with other granulomatous conditions andso is not absolutely specific for sarcoidosis. Fourper cent of untreated Hodgkin's patients, 16% ofuntreated primary biliary cirrhosis and 10% of un-treated pulmonary tuberculosis patients had raisedserum ACE activity. In this personal series no

Serum ACE activity in a number ofconditions expressed as

the percentage with raised activity (> 2SD from the controlgroup mean) from 12 International Centres.(Presented at IXth International Conference on Sarcoidosisand other granulomatous disorders. Paris 1981).

Condition Total No Serum ACE raised

No Falsepositive %

Tuberculosis 140 5 3-6Extrinsic allergic alveolitis 22 3 14Leprosy (86) 95 32 34Coccidioidomycosis (86) 13 1 7Primary biliary cirrhosis 71 19 27Hodgin's 90 3 3Beryllium disease (89) 4 3 75Asbestosis 26 3 11Silicosis 19 8 42Primary lung cancer 315 2 &6COPD 209 1 &5Asthma 86 1 15Diabetes mellitus (87) 265 48 18Alcoholic liver disease (88) 151 43 28 5Hyperthyroidism (90) 21 17 81Total 1527 189 12

patient with leprosy or inflammatory bowel diseaseshowed increased serum ACE activity (Fig. 2).

Provided due caution is taken in interpreting theresult, a demonstration of raised serum ACE inpatients with strongly suspected acute sarcoidosis canbe diagnostic, or at least provide supporting evidencethat will supplement other diagnostic procedures.Serum ACE measurements are especially helpful intwo other situations. Firstly, in patients with suspect-ed chronic sarcoidosis the Kveim-Siltzbach test ispositive in only a minority of cases and in this situ-ation a raised serum ACE supports the diagnosis andsuggests that the disease is active. Secondly, inpatients whose clinical presentation and chest radio-graph fail to suggest any particular diagnosis a raisedserum ACE will provide a lead for additional in-vestigations.The diagnostic usefulness of serumACE as a test in

sarcoidosis was confinmed at the IXth InternationalConference on Sarcoidosis (Paris 1981). The com-bined experience for 12 centres of "false positive"

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Angiotensin-converting enzyme and its clinical significance - a review

Total Totalsarcoid sarcoid sberssteroids (no steroids) cu

42 .30 57. 17 38-13

35 111 22

CO 05 cO 001 NS

Fig 2 Serum angiotensin-convertingenzyme (SACE) activity in control,sarcoidosis and other patient groups.NomWal range in shading(mean + 2 SD) statistical analysisby Mann Whitney Rank sum test.

values in conditions apart from sarcoidosis is sum-

marised in the Table. The raised values seen in otherconditions reduce the specificity. It is thereforeappropriate to consider how often a raised serumACE will accurately predict a diagnosis of sar-coidosis. The combined data showed that sarcoidosiswas present in 90% of subjects with raised serumACE (the positive predictive value). However, some40% of patients with active sarcoidosis have normalserum ACE, indicating that normal enzyme activitydoes not exclude the diagnosis.

Serial serum ACE measurements in sarcoidosisWhile a raised serum ACE is diagnostically useful, ofeven more importance are serial measurements togive advance warning of changing activity. Relativelyfew longitudinal studies of converting enzyme activ-ities in sarcoidosis have been reported9'-94 but it isclear that raised activities fall to normal should thedisease spontaneously remit or when corticosteroidssuppress activity. Rising activities may herald arelapse. The course of one patient is summarised inFig. 3. Converting enzyme activities closely parallel

clinical and radiographic changes and are helpfulin determining the adequacy of glucocorticoidtherapy.9'Apart from the clinical and radiographic features

does serum ACE correlate with other findings insarcoidosis? No significant correlation has yet beendemonstrated linking the enzyme with characteristichaematological or biochemical findings, and even

hypercalcaemia may be found with normal activity.The findings obtained by Gallium 67 Lung Scanningor bronchoalveolar lavage correlate well with theintensity of pulmonary sarcoidosis, but poorly withmeasurements of serum ACE activity.95

THERAPEUTIC AGENTS INHIBITING ANGIO-TENSIN CONVERTING ENZYMECurrent attention focuses on the renin angiotensinsystem in relation to mechanisms controlling bloodpressure and renal function. ACE inhibitors haveclinical antihypertensive properties. The role of reninin essential hypertension has been the subject of areview by Atlas and Case.' They provide convincingevidence that ACE inhibitors can be used as phar-

Control

Mean tSD 34±9r 80p Norma.

Acutesarcold

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Infamm-motoryboweldisease

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Studdy, Lapworth, Bird

Female age 32stage 11 CXRFebrile yrriphadenopathy

Fig 3 Changes in SACE activity with

Prednisa*steroid therapy are shown in a

gven 32-year-old Jamaican woman. RisingSACE was associated with clinicaland radiographic deterioration.

_-- 20mgI

Prednione - 10mg

1 _ ~~~~~~~~~~~~~~~~~~I

1 3 6 9 12Time (months)

macological probes for assessing the renin system inhypertension. A synthetic nonapeptide SQ 20881,97was the first converting enzyme inhibitor to be testedin man and early clinical studies provided evidencethat inhibition of the renin angiotensin system mightbe beneficial in hypertension.98 This has led to thedevelopment of SQ 14225 (Captopril) the first orallyeffective and apparently specific inhibitor of the con-

verting enzyme.99 Captopril causes long-terminhibition of angiotensin II formation and it hasbeen shown to be effective in lowering bloodpressure.10`03 The use of orally active ACEinhibitors has enabled the role of renin in long-termblood pressure control in essential hypertension to beinvestigated. The degree of inhibition of ACE byCaptopril can provide an indirect method of assessingthe activity of the drug. However, Roulston'" foundthat although Captopril is a potent inhibitor of serumACE, the serum had to be assayed within a fewhours. In samples stored at -20°C progressively lessinhibition was observed as the time of storage was

increased. Kamoun'05 using lower temperatures,-80°C or - 196°C found the binding of Captopril toACE stable for 12 days.

OTHER CONDITIONS AND SERUM ACE ACTIVITYThe brush border of kidney tubules is rich in ACEactivity and the measurement of urinary ACE hasbeen used by Baggio'" to provide an index of renaltubular damage. Lieberman found that raised serumACE values correlated strongly with the presence ofsevere retinopathy in diabetes mellitus and con-

sidered that vascular damage may release the enzymeinto the blood. Microangiopathy has been noted toaccompany retinal choroidopathy in sarcoidosis andhe suggests that raised serum ACE activity in sar-

coidosis may have a dual origin; from the sarcoidgranuloma and sites of microangiopathy.

Mention must also be made of one rare chronicfamilial disorder. Very high enzyme activities are

found in the serum of most patients with Gaucher'sdisease.'07 The common denominator betweenGaucher's disease and those granulomatous diseases,including sarcoidosis, in which serumACE is raised isthe macrophage which is transformed into eitherstorage or secretory cell respectively.

Finally, a number of studies in animals havedemonstrated fluctuations in ACE activity afterexperimentally induced lung damage and recentlyDobular et al, 10 working with rabbits, have reportedreduced in vivo ACE after hypoxic lung injury evenbefore light or electron microscopy histologicalchange could be detected.

Further work will need to consider the influence ofdiffuse vascular or pulmonary injury on enzymeactivities and its measurement may provide an in-direct guide to the extent of damaged endothelialsurfaces.

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Cushman DW, Cheung HS. Concentration of angiotensin-converting enzyme in tissues of the rat. Biochirn Biophys Acta1971 ;250:261-5.

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Requests for reprints to: Dr R Bird, Biochemistry Dept.,Royal Northern Hospital, Holloway Road, London N76LD, England.

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