**4.3. Classification of Glucose-6-Phosphate Dehydrogenase variants**

More than 400 variants of G6PD have been distinguished based on their biochemical characteristics, enzyme kinetics, physicochemical characteristics, and other parameters (Luzzatto & Battistuzzi, 1985, Chen*, et al.*, 1991, Greene, 1993). G6PD B+ is the most commonly found enzyme type and it is used as a standard for normal enzyme activity and electrophoretic mobility. For identification of other variants, G6PD B+ is used. The rate at which NADP+ is reduced by glucose-6-phosphate with G6PD B+ as the catalyst is the standard for activity. Based on this, enzyme activity relative to G6PD B+ variants are classified as fast, normal, and slow in terms of electrophoretic mobility and as Classes I—V (Luzzatto, 1989, Beutler, 1990, Greene, 1993, Segel, 2000, Betke K, Brewer GJ, Kirkman HN, Luzzatto L,Motulsky AC, Ramot B, and Siniscalco M 1967). There are 5 classes for these variants. Class I includes chronic nonspherocytic hemolytic anemia with a severe enzyme deficiency (e.g., G6PD Minnesota, G6PD Tokyo, G6PD Campinas). Class II variants have severe enzyme deficiency without chronic nonspherocytic hemolytic anemia (e.g., G6PD mediterrian, G6PD Canton, G6PD Union, G6PD Kaiping). Class III variants includes medium or mild enzyme deficiency, with the activity at 10-60% of G6PD B+ (e.g., G6PD Aˉ). Class IV variants have a weak or no enzyme deficiency. The activity is 60-100 % of G6PD B+ (e.g., G6PD A+). Class V variants have increased enzyme activity (e.g., G6PD Hektoen) (Beutler, 1994, Segel, 2000).
