2 IntroductionG6PD deficiency is an allelic abnormality which is inherited in an X-linked recessive fashion.G6PD deficiency is also known as "favism" since G6PD deficient individuals are also sometimes allergic to fava beans.Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is the most common human enzyme deficiency in the world; it affects an estimated 400 million people.
3 When someone has G6PD deficiency, complications can arise; hemolytic anemia and prolonged neonatal jaundice are the two major pathologies associated with G6PD deficiency.Both of these conditions are directly related to the inability of specific cell types to regenerate reduced nicotinamide adenine dinucleotide phosphate (NADPH); this reaction is normally catalyzed by the G6PD enzyme.
4 In G6PD deficient individuals, anemia is usually caused by certain oxidative drugs, infections, or fava beans.When any one of these agents, or their metabolites, enters a G6PD deficient red blood cell, hemoglobin becomes denatured, thus destroying its function as the principal oxygen carrying molecule.In addition to being susceptible to hemolytic anemia, G6PD deficient individuals are also predisposed to prolonged neonatal jaundice, this can be a potentially serious problem as it can cause severe neurological complications and even death.
5 PrincipleGlucose-6-phosphate dehydrogenase (G6PDH, D-glucose-6-phosphate) catalyzes the first step in the pentose phosphate shunt, oxidizing glucose-6-phosphate (G-6-P)to 6-phosphogluconate(6-PG) and reducing NADP to NADPH, which illustrated by the following equation:G-6-P + NADP+ G-6PDH PG + NADPH +H+NADP is reduced by G-6-PDH in the presence of G-6-P. The rate of formation of NADPH is directly proportional to the G-6-PDH activity.
6 Principle Cont.Production of a second molar equivalent of NADPH by erythrocyte 6-phosphogluconate dehydrogenase (6-PGDH) according to the reaction :6-PG + NADP+ G-6PDH Ribulose-5- phosphate + NADPH + H+ + CO2Specimen Whole blood collected with EDTA, or acid citrate dextrose .
7 Stability and StorageRed cell G-6-PDH is stable in whole blood for one week refrigerated (2-8ºc), but is unstable in red cell hemolysates.
9 Principle G-6-phosphate + NADP 6-phosphogluconic acid + NADPH Glucose -6-phosphate dehydrogenase present in the red blood cell hemolysate, act on glucose -6-phosphate and reduces NADP to NADPH which, with the help of PMS, reduces blue colored 2,6 Dichlorophenol Indophenols into a colorless form. The reaction can be represented as:G-6-phosphate + NADP phosphogluconic acid + NADPHRate of declorization is directly proportional to the activity of G-6-PD.NADPNADPH+Reduced DCPIP (colorless)2,6 Dichlorophenol indophenols (DCPIP)(Blue color)
10 NoteFresh blood sample should be use since refrigeration reduces the enzyme activity.Heparin sample should not be use as interfere with enzyme reaction.Avoid exposure of substrate vial to the light (it is photosensitive).
11 ProcedureStep1: Preparation of red cell hemolysatePurified water : 2.5mlFresh blood : 0.05mlMix well and allow standing for 5min at R.T.Step2: Assay of the enzymeAdd 1mi of the hemolysate (step 1) to the vial of solution 1 and mix gently.Add immediately about 1ml of reagent 3.Seal the vial with aluminum foil and incubate in water bath at 37ºc.
12 Procedure Cont. Observe: the time taken for the color change from initial deep blue to reddish purple. Follow up to a maximum of 6 hours with 30 min intervals.Results:Normal : min.G-6-PD deficient (heterozygous males, homozygous female): 140min-24hrG-6-Pdcarriers (heterozygous females): min-several hours.
14 PrincipleNADP is reduced by G-6-PDH in the presence of G-6-P. The rate of formation of NADPH is directly proportional to the G-6-PDH activity and is measured spectrophotometrically as an increased in absorbance at 340nm.
15 Procedure1- Sample PreparationWash 0.2 ml of blood with 2 ml aliquots of 0.9% NaCl solution.Centrifuge after each wash for 10 min at around 3000 rpm.Repeat 3 times.Suspend the washed centrifuged erythrocytes in 0.5 ml of solution 4 and let stand for 15 min at +4°C and then centrifuge again.Use the supernatant in the assay within 2 hours.
16 Procedure Hemolysate 15 µl R1 (buffer) 1000 µl R2 (NADP) 30 µl 2- Enzyme assayA-Add the following reagentsHemolysate µlR1 (buffer) µlR2 (NADP) µlB- Mix incubate 5 min at 370CC- Add 15 µl R3 (Substrate G6P) Mix and read the initial absorbance at 340 nm against air(A1)D- Read again after 1, 2 and 3 minutes.(A2, A3, A4)
17 Calculation:To determine G-6-PDH activity, do the following calculations:ΔA per min =( Afinal - Ainitial )G-6-PDH activity is expressed as U/1012 erythrocyte (RBC)or as U/g hemoglobin (Hb).
18 G-6-PDH (U/1012 RBC)G-6-PDH (U/1012 RBC) =( ΔA per min X60571) / erythrocyte countG-6-PDH(U/g Hb) = ΔA per min X60571 X100/Hb (g/dl)ExampleΔA = 0.011RBCs count 5.3 x 10 12Hb = 15G-6-PDH (U/1012 RBC) = x 60571/5.3 = 125G-6-PDH(U/g Hb)= x x 100/15 = 4.442
19 NoteIf anemia and/or leukocytosis is presentUse Buffy coat free blood sample for assay (platelets and WBCs marked activity in this enzyme)Normal rangeG-6-PDH (U/1012 RBC): ( )G-6-PDH (U/g Hb): ( )