Historically, the name refers to the property of some Hb to precipitate when incubated at 50 °C for 1 h, unlike HbA, which remained stable. Subsequently, new tests were developed to assess stability, such as incubation at higher temperatures (up to 65 °C) and kinetic assessment of stability. One test that has proven to be very valid for assessing the stability of Hb is the isopropanol test, which, however, should be considered parallel to and not a substitute for traditional heat tests.

The instability is caused by structural changes that allow water (H₂O) access to the heme (hydrophobic) pocket, resulting in the development of free radicals, denaturation, and precipitation of the molecule.

Typically, it is the introduction of proline or glycine into an Hb helix that disrupts the secondary structure and causes instability and precipitation with the detachment of the heme. Structural abnormalities can be close to the heme or in the contact area between globin and heme or be due to insertion or deletion (Fig. 1). The α, β, and γ globins are affected.

Fig1. Molecular abnormalities causing hemoglobin instability. Next to each type of abnormality are the names of some better-known unstable hemoglobins. (Copyright EDISES 2021. Reproduced with permission)

Of the approximately 250 mutations that affect globin chains and can reduce the stability of the molecule, roughly half are important because of the clinical consequences of the variant. Furthermore, since instability is caused in only a portion of the variant hemoglobin molecules, few Hbs are detectable by traditional methods. Mutations can cause four conditions:

• Instability detectable only in vitro, without clinical- hematological consequences.

 • The destruction of the most unstable Hb within minutes/ hours of synthesis causes ineffective erythropoiesis and thalassemia-like syndrome. They should be identified by molecular biology methods.

 • The intermediate condition between the two previous ones, with a variant present in erythrocytes where it precipitates under conditions of oxidative stress.

 • Dominant phenotypic characteristics due to de novo mutations.

 From a clinical point of view, they are present at varying degrees depending on the type of variant:

• Congenital nonspherocytic hemolytic anemia with splenomegaly and cholelithiasis (bilirubin stones)

• Hemolytic anemia with Heinz bodies, with sensitivity to oxidizing drugs such as sulfonamides

• Modest/slight anemia with reticulocytosis disproportion ate to the level of circulating hemoglobin

• Peripheral hematological picture similar to thalassemia with hypochromic erythrocytes

 • Increased methemoglobin formation

Among the most known variants, Hb Koln (β 98 Val → Met) has been described in different groups and geo graphical locations; Hb Hasharon (α 47 Asp → His) is pre dominantly described among Ashkenazi Jews where it causes neonatal hemolysis; unstable γ variants, such as Hb Poole (γ 130 Trp → Gly), are associated with hemolysis in the first months of life and then disappear as the synthesis of γ chains decreases within the first year of life; on the contrary, children with unstable β variants appear normal at birth, with progressive hemolysis developing during the first year of life as a result of increased synthesis of β chains. Unstable α variants differ depending on whether the affected gene is α1 or α2, which encodes for higher levels of mRNA. Consequently, a mutation in the α2 gene has a stronger clinical impact than that in α1, resulting in classic unstable Hb syndrome and syndrome with thalassemia-like phenotype with intramedullary Hb destruction. Some forms of HbH (β4) are due to the unstable α gene. These forms, not being of thalassemic nature, should be considered during genetic counseling. The main unstable hemoglobins are shown in Table 1.

Table1. Main unstable hemoglobins

The diagnosis of unstable hemoglobin should be made based on the following:

 • Regenerative hemolytic anemia with increased reticulocytosis.

• Heinz bodies (not always pathognomonic as they are also present in erythroenzymopathies and following the intake of certain drugs).

• Anisocytosis, basophilic punctuation, and macrocytosis with reduced MCH.

 • Separation procedures (electrophoresis, HPLC). However, they do not always allow identifying variants unless they are mutations causing altered HPLC mobility and/or electrophoretic migration.

 • The presence of hemichromes, semi-Hb, or free α-chains, may indicate the presence of unstable β-chain.

• Combinations of methods and use of HPLC combined with MS.

اخر الاخبار

اخبار العتبة العباسية المقدسة

يجسد قيم النهضة الحسينية.. قسم الشؤون الفكرية يصدر العدد 120 من مجلة (حيدرة) الثقافية

ضمن حملة الكفيل العتبة العباسية المقدسة تحيي مراسم ليلة الحادي عشر من المحرم في سامراء

المجمع العلمي يحيي عاشوراء بخدمة الزائرين وإقامة مجالس العزاء ببغداد والحمزة الشرقي

المؤمنون يحيون ليلة الحادي عشر من المحرّم عند مرقدي الإمام الحسين وأخيه العباس (عليهما السلام)

المزيد

الآخبار الصحية

تناول الجبن.. مفيد للجسم أم خطر يهدد صحتك؟

طبيبة أسنان تحذر.. لا تتجاهل هذه العلامة بعد التنظيف

يجب أن تأكل المزيد من الشمام هذا الصيف.. إليك السبب ؟

تأثير صحي كبير.. كم بيضة يجب أن تتناول يوميا؟

المزيد

الآخبار التكنلوجية

أداء خلايا البيروفسكايت الشمسية تحت الماء قد يزيد كفاءة تحويل الكهرباء

مسافة الأمان: أساس القيادة الآمنة وتجنب الحوادث

هل القيادة السريعة تعني استهلاك اكثر للوقود؟

لماذا يعتبر كوكب المريخ غير صالح للحياة؟

المزيد

مواضيع ذات صلة

C-Reactive Protein Rapid Latex Agglutination Test

Hyporegenerative Macrocytic Anemias/ Megaloblastic Anemias

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

Hereditary Spherocytosis

Magnesium

lyme disease test

luteinizing hormone (LH assay, Lutropin) and follicle stimulating hormone (FSH) assay

liquid biopsy (Fluid biopsy, Fluid phase biopsy)

lipoproteins (High-density lipoproteins [HDLs, HDL-C], Low density lipoproteins [LDLs, LDL-C], Very-low-density lipoproteins [VLDLs], Lipoprotein electrophoresis, Lipoprotein phenotyping, Lipid fractionation, Non-HDL cholesterol, Lipid profile)

lipoprotein-associated phospholipase A2 (Lp-PLA2 PLAC test)

Hemolytic Anemias

Sickle Cell Anemia (Hemoglobinopathy S, Sickle Cell Disease)