J Clin Pathol: first published as 10.1136/jcp.36.6.697 on 1 June 1983. Downloaded from J Clin Pathol 1983;36:697-700 A fluorimetric method for red blood cell sorbitol dehydrogenase activity G VACA, P ZIJIGA, C MEDINA, R ALONSO, G GONZALEZ-QUIROGA, RI ORTIZ-DE-LUNA, JM CANTt From the Division de Genetica, Subjefatura de Investigaci6n Cientifra, Unidad de Investigacion Biomedica, Centro Medico de Occidente, Instituto Mexicano del Seguro Social, Apartado Postal 1-3838, Guadalajara, Jalisco, Mexico SUMMARY A new fluorimetric method for the quantification of red blood cell (RBC) sorbitol dehydrogenase is described. It is based on the oxidation of sorbitol to fructose, in presence of NAD+, catalysed by the RBC-sorbitol dehydrogenase. The quantity of NADH formed is then measured in a filter fluorimeter. Comparison with an indirect spectrophotometric assay yielded good correlation; however, the present method offers several advantages: it is more rapid, simple and inexpensive. It should be useful to screen for sorbitol dehydrogenase deficiency in large numbers of individuals, particularly patients with diabetes or cataracts. The sorbitol pathway involves two enzymes: aldose methods, both spectrophotometric assays, for reductase (EC 1.1.1.21) and sorbitol dehydrogenase RBC-sorbitol dehydrogenase activity have been copyright. (EC 1.1.1.14). Aldose reductase catalyses the con- previously described.2 7 In one of them,2 the back- version of glucose and galactose to sorbitol and ward reaction from fructose to sorbitol is directly galactitol, respectively, whereas sorbitol dehydro- estimated, however, the RBC sorbitol dehydrogen- genase converts sorbitol to fructose: ase activity is only about 30% of the activity in the forward direction.7 The other method7 is an indirect Aldose reductase Sorbitol dehydrogenase two-stage assay designed to estimate activity in Glucose Sorbitol b-fructose haemolysates, whose high optical density precludes http://jcp.bmj.com/ NADPF'+ H~+ NADP+ NAD+ NADH + H+ direct optical assay of the enzyme. Although this in assay is adequate for the measurement of RBC- The importance of this pathway "sugar-cataract" sorbitol dehydrogenase activity, it is expensive and formation has been well established since sorbitol time-consuming. Since we are interested in the and galactitol are involved in the pathogenesis of diabetic and galactosaemic cataracts, respectively, in quantification of sorbitol dehydrogenase activity in both experimental animals and humans (for review, large number of individuals, we sought for a rapid, simple and less expensive assay. In the new assay on September 27, 2021 by guest. Protected see van Heyningen).' Since human red blood cells described in this paper, the NADH formed as a (RBC) have the metabolic capacity for glucose- result of the sorbitol to fructose oxidation in pres- sorbitol-fructose conversion,2-7 the screening for ence of NADI and catalysed by RBC-sorbitol inborn errors of the sorbitol pathway, particularly for sorbitol dehydrogenase deficieny in patients with dehydrogenase is measured in a filter fluorimeter. hyperglycaemia or cataracts, seems quite attractive. This inborn error could lead to sorbitol accumula- Material and methods tion in the lenses, among other tissues, and eventu- ally to cataracts, like galactokinase or galactose-1- The composition of the assay mixture for the phosphate uridyltransferase deficiencies lead to fluorimetric method is based with slight modi- cataracts through galactitol accumulation. The diag- fications, on the one used in the first-step of the nosis of both galactokinase and galactose-1- two-stage spectrophotometric assay of Barretto and phosphate uridyltransferase deficiencies is carried Beutler.' One ml of this mixture is prepared by mix- out in RBC.8 ing 100 ,Il 1 M, Tris-HCl, pH 8-0; 100 ,ul 50 mmol To the best of our knowledge, two quantitative NAD+; 100 ,ul 200 mmol MgCl2; 500 ,ul 200 mmol sorbitol; and water to a total volume of 1-0 ml. Accepted for publication 6 January 1983 Blood collected in heparin was obtained from pro- 697 J Clin Pathol: first published as 10.1136/jcp.36.6.697 on 1 June 1983. Downloaded from 698 Vaca, Zdn-iga, Medina, Alonso, Gonzdlez-Quiroga, Orttz-De-Luna, Canid fessional blood donors. The RBC were washed three 10 x A C= times in cold saline solution and the buffy coat was F1 removed. The red cells were diluted with two vol- Barretto and Beutler spectrophotometric assay foi umes of cold water and haemolysed by freezing in a sorbitol dehydrogenase activity7 in blood samples dry-ice-acetone bath and thawing, for three times. obtained from 50 professional blood The fluorescent assay was carried out in these donors was lysates. carried out for reference purposes. Results PROCEDURE The procedure was similar to the Beutler's Sorbitol dehydrogenase in erythrocytes is stable for fluorimetric method for galactose-1-phosphate uridyltransferase activity.9 The assay is carried out 40- by placing 200 ,ul of the substrate mixture in a test .0 0o tube (13 x 100 mm) with the addition of 20 ul of I a, 30- haemolysate followed by incubation at 37°C for 60 0,0 min. A blank system without sorbitol was included E0 for each sample. The reaction was stopped by dilut- 0~~~~~ ing 20 of the incubation mixture in 4 ml of 0.01 M 20- ,lp 0~~~~~ potassium phosphate buffer pH 7*4. The diluted 0I incubation mixture is transferred to a 10 mm (inner 0~~~~~0/ diameter) round Turner photofluorimeter with a - 10- Turner No 110-811 (7-60) primary and No 110- 0o 817 (8) secondary filters. The fluorimeter was adjusted to read zero with the dummy cuvette. The 0 15 30 45 60 75 90 105 120 fluorescence of the blank system was subtracted Time (rrin) from the fluorescence of the complete system. The Fig. 1 The effect ofincubation time on the enzyme copyright. diluted reaction mixture was then transferred to a velocity. The reaction system was prepared as described in 0 5 by 4 inches (1 1-60 mm inner diameter) Bausch the text using haemolysate diluted 113 (Hb 7-10 g%) and and Lomb Spectronic 20 colorimeter- incubated for two hours. Aliquot parts were removed from spectrophotometer cuvette, and the optical density the incubating mixture each 15 min and the ,mol product was determined at 410 nm against a buffer blank to formed per g Hb estimated as stated in the text. Each point represents the mean value ofan assay for triplicate. estimate the Hb content. Sorbitol dehydrogenase http://jcp.bmj.com/ activity (E) in ,umol of product formed per gramme 0.04 of Hb per hour is obtained as follows: C E - FC 0w A E .' 0*03 where F = the net fluorescence of the sample; U' unU' C = the calibration factor; c = at nm. 0 A the absorbance of the diluted system 410 u on September 27, 2021 by guest. Protected Calibration of the method was carried out according _ 002 to the procedure used by Beutler and Mitchell9 in the fluorimetric assay for galactose-1-phosphate uridyltransferase. Serial dilutions of a solution of known NADH concentration were prepared and fluorescence determined in the fluorimeter; serial -3 dilutions of the haemoglobin solution of known con- E centration were prepared and absorbance deter- mined at 410 nm in the colorimeter. The fluores- 0 0.001 0-002 0-003 cence readings were plotted against the NADH con- (g Hb/reoction system) centrations and the absorbance readings against Fig. 2 The effect ofthe enzyme activity on the enzyme haemoglobin (g/l). The fluorescence reading given velocity. The reaction systems wereprepared as described in by 1 pLmol NADH is read from the respective curve the text using different dilutions ofa haemolysate and and designated F. The absorbance given by a 0 1 g/l incubated for one hour. Both the pmolproduct and the Hb solution of haemoglobin is read from the curve and concentration by reaction system were estimated from is designated A1. The calibration factor (C) is calcu- NADH and Hb calibration curves respectively. Each point lated as follows: represents the mean value ofan assay for triplicate. J Clin Pathol: first published as 10.1136/jcp.36.6.697 on 1 June 1983. Downloaded from Sorbitol dehydrogenase measurement 699 D I ised.7 Furthermore, by hybridisation of human and hamster cells the locus of sorbitol dehydrogenase 0 20 0 0 0 to chromosome 15 o 0 0 has been provisionally assigned 0 o at the pter- q21 band."' However, little is known 0 ~~~000 0 about the biochemical genetics of human sorbitol Cb e 0 0000 dehydrogenase. Charlesworth has studied red cell 0 0 sorbitol dehydrogenase in 665 individuals and ~~0 0 variant.'2 Our 0 one rare 0 reports only electrophoretic 10- 0 0000 group has demonstrated electrophoretic polymorph- £.E °___<E?__0 ° 50 ism of the enzyme in human seminal plasma'3 and 0 0 81 RBC-sorbitol dehydrogenase deficiency in a family a ~~0rPo :0 001 with congenital cataracts.'4 This enzymatic defect was detected by a qualitative fluorescent screening 0 10 20 30 test later confirmed by both enzymatic quantitative Spectrophotometric method (,umol product/h/g Hb) assays and methaemoglobin reduction by intact RBC incubated in presence of sorbitol.'4 The results Fig. 3 Correlation between the quantitative fi.sorescent method and the Barretto and Beutler spectrophotometric did not define a clear cataract-sorbitol dehydrogen- two-stage assay. Each point represents the mean value of ase deficiency aetiopathogenic relation but activity assays performed by duplicate. polymorphism in RBC-sorbitol dehydrogenase was very likely. This finding would be highly relevant not at least five days when whole blood is kept at 4°C. only to the study of cataracts but of other major The enzyme tests were repeated at least twice within complications in diabetes, such as neuropathy and five days in several samples.