In summary, the reduced activity of G6PD variants observed in red
blood cells may originate from several causes. Reduced catalytic effi-
ciency and protein stability are clearly the major causes of clinical en-
zyme deficiency observed in individuals carrying G6PDViangchan and
G6PDViangchan + Mahidol. From detailed biochemical characterization, we
report here, for the first time, that G6PDViangchan exhibited a 10-fold de-
crease in catalytic efficiency and this variant also showed lower thermo-
stability when compared with the native enzyme. This result further
supports the classification of this variant as a Class II G6PD deficiency.
Moreover, the double mutant enzyme, G6PDViangchan + Mahidol, displayed
an even greater loss in catalytic efficiency (18-fold) with a significant re-
duction in thermostability when compared with G6PD WT. This double
mutant, therefore, should be classified as a severe G6PD deficiency.
However, clinical data is also required to precisely allocate it to a Class
I or Class II G6PD deficiency. Thus, functional characterization of both
G6PD variants has provided valuable information describing the molec-
ular mechanisms underlying the severity of clinical manifestations.