The molecular weight and subunit structure of glucose‐6‐phosphate dehydrogenase from human erythrocytes

Glucose&phosphatedehydrogenase (G6PD; EC 1 .l.1.49)catalyses the initial reaction of the pentose phosphate pathway, and thus its function is necessary in maintaining the level of the reduced coenzyme NADPH in various tissues.Human G6PD exhibits polymorphism, the structural gene being on the X-chromosome [ 11.The variant forms differ from the normal enzyme in electrophoretic mobility or in catalytic activity [2,3].While deficiencies in function do not appear to affect other tissues, most variants with low activity are associated with haemolytic anaemias, either chronic or induced by administration of drugs or ingestion of toxic substances.In the red blood cell, NADPH is used to reduce glutathione [4], which is required to preserve sulphydryl groups and so keep the cell intact.On the basis of kinetic measurements, it has been suggested that catalysis by G6PD in the red cell and in other tissues may be subject to metabolic control [S-l 01.In studying this possibility, or the extent of such control, it is necessary to know the molecular size and number of subunits of the enzyme.Previous molecular weight measurements of the normal erythrocyte enzyme, termed Gd(+) B [2], varied between 105,000 [ 1 l] and 240,000 [ 121.Yoshida concluded from the molecular weight in 4 M guanidium chloride that the enzyme contains 6 subunits [ 123, and from the number of tryptic peptides that the subunits were identical [ 131.In the present investigation of the enzyme, the sedimentation coefficient was found to vary with the solvent conditions.The data also indicate that more than one dissociation equilibrium was occurring.It is necessary to separate these equilibria to establish the molecular size and subunit content of the protein.