Hemoglobin Alc and Diabetes Mellitus

Transcription

1 ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 10, No. 2 Copyright 1980, Institute for Clinical Science, Inc. Hemoglobin Alc and Diabetes Mellitus HYMAN ROCHM AN, P h.d., M.D. Department o f Pathology, University o f Chicago, Chicago, IL ABSTRACT HbAj is the result of the non-enzym atic glycosylation of HbA. HbAt reflects the integrated blood glucose level that prevailed over a period of w eeks; it thus has clinical application in the assessm ent of the long-term blood glucose control. This ability to determ ine easily the long-term blood glucose should enable the clinical investigator to gain insights into the relationship of glycem ia and th e com plications resulting from long-standing d iab etes m ellitus. Introduction D iabetes m ellitus is one of the m ost com m on diseases, affecting one to five percent of the total population. It is estim ated th at in the U nited States alone, over 200,000 new cases are discovered every year. T he treatm ent of diabetes aims to norm alize fat, protein and carbohydrate m e tabolism. In the overt diabetic, control of excessive fatty acid oxidation and m obilization takes p re c e d e n c e, b u t lim itin g hyperglycem ia, including urinary caloric loss, and restoring protein losses are also v ital considerations. In contrast, the long term treatm ent of diabetes has em phasized control of the blood glucose level not only to p revent th e acute com plications of ketosis and h y p e rg ly c e m ia b u t also p e rh a p s in m inim izing long-term diabetic com plications. It is to this latter aspect, the relation b etw e en glucose levels and long-term 111 diabetic com plications, that HbA! levels m ay prove to be of considerable use. O ne o f th e m ajor problem s in d eterm in in g w hether or not the long-term com plications of diabetes is glucose-related has b een the lack of a m easure of long-term glucose control. T he findings that HbAx is a satisfactory indicator of th e integrated blood glucose level over prolonged p e riods of tim e has thus aroused considerab le interest. T he m ajor form, over 90 p ercen t, of adult hem oglobin is HbA (HbAu ) w ith a subunit structure of a 2/32. M ost of the rem ainder consists of HbAl0 (1 to 2 percent), HbA16 (1 to 2 percent) and HbAlc (4 to 6 percent). T hese latter m inor hem oglobins can be separated from HbA by various chrom atographic procedures. At a slightly acidic ph, the m inor hem oglobins are m ore negatively charged than HbA. Thus, w hen hem olysate is applied to a cation exchange resin elu ted w ith buffer, p H 6.7, th e first fractions w ill co n ta in H ba la / $00.90 Institute for Clinical Science, Inc.

2 1 1 2 ROCHMAN w hich is follow ed by HbA16 an d later HbAlc. HbAlc was first identified by Allen et a l.1 It w as fo u n d b y H o lm q u is t an d S chroeder16 to differ from H ba by an unident fied com pound b o u n d to the term i nal n -ogen o f th e fi -chain. Bookchin and Gallc 3 th en show ed th at th e N -term inal com p iund of th e )3-chain possessed an added hexose, identified by B unn e t al4 as glucose and m annose in a ratio of 3 :1. G ly c o sy la tio n is n o t lim ite d to th e N -term inal position o f the /3-chain, since it is also know n to link to the e-am ino groups of lysine residues in both a and /3 chains.12 It has b een show n that HbAle is the result ' th e glycosylation o f H ba th e reaction occurring in the erythrocyte during its stay in the peripheral circulation.10 T h e g ly c o sy la tio n re a c tio n is nonenzym atic, and th ere is convincing ev i dence that its rate is d ep e n d en t on the blood glucose level and the duration of exposure to b lo o d glucose. T h u s, the HbAic content of aged erythrocytes is significantly h ig h er than that from young erythrocytes an d this was observed for both healthy subjects and d iab etic patients. Sim ilarly, th e lev el of H ba lc is low er in patients w hose erythrocytes have a shortened life span ow ing to hem olysis.3 T h e increased glycosylation of hem o globin u nder conditions of a raised blood glucose is m ost easily observed in the accu m u latio n o f H ba lc, th is b e in g th e m ost ab undant of the m inor hem oglobins. H ow ever, an increase in glycosylation of the e-am ino lysine residues o f the a and chains has also b een noted. In addition to the findings of an increase in glycosylated hem oglobin, changes in other glycoproteins have b een rep o rted in d ia b e te s. T h e re is e v id e n c e th a t th e th ick en ed b asem en t m em brane of diabetic glom eruli is com posed of abnorm al glycoprotein selectively en rich ed w ith c a rb o h y d ra te.2 F u rth e r, q u a n tita tiv e changes in fibrinogen and von W illebrand factor, also g ly co p ro tein s, o ccu r w ith d iab etes.7 HbAlc and D iabetes M ellitus T he findings of increased am ounts of a m inor hem oglobin com ponent in diabetes a n ted a ted th e id en tificatio n o f HbAlc. H uism an and D ozy16 in 1962 w ere the first to report a 2 to 3 fold increase in the m inor hem oglobin com ponent in four diabetics. An in d ep en d ent study by R ahbar18 also found an elevation in a m inor hem oglobin in two diabetics, and he later confirm ed this in 47 others. T he m inor hem oglobin com ponent was su b seq u en tly identified as HbAic. Am ongst the early clinical studies of HbAlc levels in diabetic patients was that by Koenig et al.17 T hey found th at the concentration o f HbAic appeared to reflect the m ean blood sugar level over the previous w eeks. C o n sistent w ith th ese findings have b e e n those of o th er investigators who found that th e highest correlation betw een H balc and urinary glucose was w ith urinary glucose excreted two months prior to the HbAj determ ination.5,11 T hese data sug gested th a t glycosylated hem o globin levels reflect th e in teg rated glucose co n c en tratio n o v er th e p revio u s weeks and hence an index o f long-term blood glucose control. T hus its potential use, for studies to determ ine w h eth er or not long-term diabetic com plications are a consequence of poor m etabolic control, becam e apparent. T here have b een no reports as y et of p ro sp e c tiv e long-term studies. T rivelli e t a l19 did not find a difference w h en iso lated HbAi levels of diabetic patients w ith or w ithout longte rm c o m p lic a tio n s w e re co m p a re d ; studies o f long-term com plications in cluded p erip h eral vascular disease, retin o p a th y, n e u ro p a th y a n d c e re b ro vascular accidents. Sim ilar results w ere obtained by C oller e t a l7 in studies on d iab etic retinopathy. Certain know n reversible biochem ical phenom ena and HbAj in diabetes have been studied. Fasting plasm a glucose and the m ean value obtained from frequent blood glucose estim ations over 24 hour

3 HEM OGLOBIN A*. AND DIABETES M ELLITUS p erio d s sh o w ed ex c ellen t correlations w ith HbAj levels.13 D iabetics are prone to a n e m ia, in fe c tio n a n d th ro m b o tic episodes w ith m acrovascular and microvascular consequences. T hese com plications may be partly due to the specific abnorm alities of leucocyte and p latelet fu n c tio n th a t h av e b e e n o b se rv e d in diabetic patients. To determ ine w hether or not these com plications w ere related to the d egree of blood glucose control, tests on the form ed elem ents of blood w ere com pared w ith HbAlc levels. It was found that the erythrocyte half-life, leucocyte adherence and the secondary lag phase tim e of epinephrine-stim ulated p latelet aggregation show ed a good correlation w ith H ba j.7 F urther, correction by strict carbohydrate control over w eeks low ered the H balc levels an d this was accom p a n ie d by a rev ersal o f th e abnorm al hem atological tests. T he m etabolic derangem ents observed in new born infants of diabetic w om en h ave b e e n a ttrib u te d to th e m o th ers chronic hyperglycem ia. Since HbAlc re flects the chronic glycem ic state, it has been studied in such pregnancies. T he results show ed that w hen corrected for gestational age, th e relative b irth w eights correlated in a significant linear m anner w ith m aternal HbAlc levels. T he authors concluded th at HbAlc levels could be of diagnostic im portance for predicting fetal size.21 Also of interest is the finding that the pregnant diabetic has a low er than exp ected concentration o f HbAlc. This has also b een observed in pregnant m onkeys (female m acaque monkeys).20 This lowered HbAlc level of pregnancy m ight be due eith er to low er m ean blood glucose values during pregnancy, a horm onal effect or a d ec rea sed erythrocyte survival. G lycosylated H em oglobin and H em oglobin Function D oubling or even tripling o f the HbAlc content o f erythrocytes, as frequently observ ed in d ia b e tic p atien ts, m ay com p ro m ise oxygen d eliv ery. D itz e l and Standi9 have show n that the P50 at the in vivo ph of w hole blood of non-acidotic d iabetic p atien ts is significantly low er th an norm al. T his decrease occurs b e cause the binding o f 2,3-diphosphoglycerate (2,3-DPG), an im portant intracellular red cell glycolytic in term ed iate, to HbAic is inhibited. This phenom enon has b een explained by steric interference of 2,3-D PG by th e N -term inal /3-chain glucose of HbAlc being at a position in the hem oglobin m olecule w here this ligand w ould norm ally b in d and exert its effect. D ietzel and Standi9 have suggested that this increase in oxygen affinity is responsible for tissue hypoxia and, if chronic, co u ld lead to long-term m icrovascular com plications. A sim ilar finding o f an increase in 2,3-DPG has been observed in m acaque m onkeys induced w ith diabetes eith er by streptozotocin-treatm ent or pancreatectom y; how ever, the increase was not considered significant and this may have b een due eith er to the sm all sam ple size or a less severe glucose intolerance.20 M ethodology T h e c u rre n t p ra c tic e in m e a su rin g HbAi0, HbAi6 and HbAlc is carried out e m p lo y in g c a tio n ex c h an g e co lu m n chrom atography. S ince a separate d e te r m ination for HbAlc probably does not offer any special advantage in the clinical s e ttin g, m o st la b o ra to rie s m e a su re HbAI; the sum of the m inor hem oglobins. T he term HbAx, encom passes the sum of HbAja 4- HbAlft + H ba lc. A d e tailed description of this m ethod has been reported by G abbay e t al and is based on a m odification of the original procedure d e scrib ed by T riv elli et al.19 C atio n exchange chrom atography is p articu larly laborious an d tim e-consum ing for a service laboratory and more rapid m ethods have b een explored. Davis e t a l8 and Cole e t al6 have described the application of high pressure liq u id chrom atography for m easu rin g HbAj. M ore recen tly, a col

4 11 4 ROCHMAN orim etric m ethod has b een em ployed and in this in stan ce th e total glycosylated hem oglobin has b een m easured; this in cludes glycosylated HbA in addition to that o f HbAj.12 Indications and Lim itations for the Use of HbAj The m easurem ent of HbAj in th e assessm ent of the diabetic p atien t has considerable advantages over p resen t tests. In contrast to 24-hour urinary glucose or isolated blood glucose determ inations, HbA! is not d ep enden t on th e accuracy of volum e collection and it is in d ep en d ent of th e tim e of day, m eals or exercise. It is convenient to b e able to describe m etabolic control w ith a single value, so that HbAi provides th e phy sician w ith a d e finitive end-point tow ards w hich therapy needs to be directed. HbAj m ay also be usefully em ployed as a screening test for those diabetics w ho appear to be satisfactory b u t whose m etabolic control is actually less than optim al.14 H o w e v er, H ba i re fle c ts c h ro n ic glycem ia so that it cannot supersede the necessity for blood glucose estim ations in acute situations. Also, HbAj levels do not reflect the occurrence o f hypoglycem ic episodes. Since th e glycosylated h em o globin in situations of a shortened erythrocyte half-life an d/o r certain hem oglobinopathies underestim ates the degree of chronic hyperglycem ia, the application of H ba j is lim ite d in p a tie n ts w ith hem olysis, /3-thalassemia or H bf. R eferences 1. A l l e n, D. W., Sc h r o e d e r, W. A., and Ba l o g, J.: O bservations on th e chro m ato g rap h ic heterogeneity of normal adult and fetal hemoglobin: A study of the effects of crystallization and chromatography on the heterogeneity and iso leu cin e co n ten t. J. Amer. C hem. Soc. 8 0 : , B e is s w e n g e r, P. J. a n d S p iro, R. G.: H u m a n g lo m e ru la r b a s e m e n t m e m b ra n e : C h e m ic a l a lte r a t i o n in d i a b e t e s m e l l i t u s. S c ie n c e 168: , BOOKCHIN, R. M. and G a l l o p, P. M.: Structure of HbAlc. Nature of the N-terminal beta-chain blocking group. Biochem. Biophys. Res. Commun. 32:86-93, B u n n, H. F., H a n e y, D. N., G a b b a y, K. H., and GALLOP, P. M.: Further identification of the nature and linkage of th e carbohydrate in hemoglobin Alc. Biochem. Biophys. Res. Commun. 67: , B u n n, H. F., G a b b a y, K. H., and G a l l o p, P. M.: The glycosylation of hemoglobin: Relevance to diabetes mellitus. Science 200:21-27, C o l e, R. A., B u n n, H. F., a n d So e l d n e r, J. S.: N e w ra p id assay m e th o d fo r h e m o g lo b in (HbAlc a n d to tal fast H b ). D ia b e te s 26 (S u p p l. 1):392, 1977 (ab stract 157). 7. C o l l e r, B. S., F r a n k, R. N., M i l t o n, R. C., a n d G r a l n ic k, H. R.: P la s m a c o fa c to rs o f p la te le t fu n c tio n : C o rre la tio n w ith d ia b e tic re t in o p a th y a n d h e m o g lo b in s Ala_c. A nn. In t. M ed. 8 8 : , D a v is, J. E., M c D o n a l d, J. M., a n d J a r r e t, L.: A h ig h p e rfo rm a n c e liq u id c h ro m a to g ra p h y m e th o d fo r h e m o g lo b in Alc. D ia b e te s 26 (S uppl. 1): 386, 1977 (ab stract 62). 9. DlTZEL, J. and S t a n d l, E.: The problem of tissue oxygenation in diabetes mellitus. Acta Med. Scand. (Suppl.) 578:49-83, F it z g ib b o n s, J. F., Ko l e r, R. D. and J o n e s, R. T.: Red cell age-related changes of hemoglobins Ala+6 and Alc in normal and diabetic subjects. J. Clin. Invest. 58: , G a b b a y, K. H., H a s t y, K., B r e s l o w, J. L., E l l i s o n, R. C., B u n n, H. F., and G a l l o p, P. M.: Glycosylated hemoglobins and long-term blood glucose control in diabetes mellitus. J. Clin. Endocrinol. Metab. 44: , G a b b a y, K. H., S o s e n k o,j. M., B a n u c h i, G. A., M in in s o h n, M. J., and F lu c k i g e r, R.: Glucosylated hemoglobins: Increased glycosylation of hemoglobin A in diabetic patients. Diabetes 28: , G o n e n, B., R u b in s te in, A. H., R o ch m a n, H., T a n e g a, S., and H o r w it z, D. L.: Hemoglobin Ai'. An indicator of the m etabolic control of diabetic patients. Lancet 2: , G o n e n, B. and R u b in s t e in, A. H.: Hemoglobin A, and diabetic mellitus. Diabetologia i5 :l- 8, H o l m q u is t, W. R. a n d Sc h r o e d e r, W. A.: A n e w N -te rm in a l b lo c k in g g ro u p in v o lv in g a S ch iff b a s e in h e m o g lo b in AJC. B io ch e m istry 5:2489 a2 2503, H u is m a n, T. H. J. a n d D o z y, A. M.: V. B in d in g o f h e m o g lo b in w ith o x id iz e d g lu ta th io n e. J. L ab. C lin. M e d. 6 0 : , Ko e n ig, R. J. and C e r a m i, A.: Synthesis of hemoglobin A,c in normal and diabetic mice: Potential model of basem ent membrane thickening. Proc. Nat. Acad. Sei. 72: , R ahbar, S.: An abnormal hemoglobin in red cells of diabetics. Clin. Chim. Acta22: , 1968.

5 HEM OGLOBIN A lc AND DIABETES M ELLITUS T r iv e l l i, L. A., Ra n n e y, H. M., and L a i, H-T.: Hemoglobin components in patients w ith diabetes mellitus. New Eng. J. Med. 284: , WIDNESS, J. A., Sc h w a r t z, R., T h o m p s o n, D., T s u b o i, K. K., R e y n o l d s, W. A., C h e z, R. A., and Sc h w a r t z, H. C.: Hemoglobin Alc in the glucose-intolerant, streptozotocin-treated or pancreatectom ized macaque monkey. Diabetes 27: , W id n e s s, J. A., Sc h w a r t z, H. C., T h o m p s o n, D., Kin g, K. C., Ka h n, C. B., and O h, W.: Glycohemoglobin (HbAlc): A predictor of birth w eight in infants of diabetic mothers. J. Pediat. 92:8-12, S c h w a r t z, H. C., Kin g, K. C., Sc h w a r t z, A. C., E d m u n d s, D. and Sc h w a r t z, R.: Effects of pregnancy on hemoglobin Alc in normal gestational diabetic and diabetic women. D iabetes 25: , 1976.