1. The molecular weights of the G6PD subunits from the three yeast sources, bovine adrenals and rat liver were estimated by SDS-PAGE and found to be in the region of 55-59 K. An estimate of 46 K was similarly made for the G6PD subunit from the prokaryote, L. mesenteroides. 2. Two close bands or sometimes one diffuse band appeared on the polyacrylamide slab gel following electrophoresis and Coomassie blue staining for rat liver G6PD. These corresponded to the molecular weight region of about 57-59 K. So far, the evidence from peptide sequences does not reveal more than one subunit sequence. 3. G6PD from bakers' yeast was inactivated with sodium [1-. C] acetylsalicylic acid, following whichit was shown that 1.1 mole of 14 C moiety had been incorporated per enzyme subunit and that a lysine residue, essential to enzyme activity, had been modified. 4. Inactivation of bakers' yeast G6PD in the presence of high concentrations of substrate or coenzyme indicated that the acetylsalicylic acid was binding to the enzyme at a site which was directly or indirectly involved in substrate binding. The observation that high concentrations of coenzyme in the incubation mixture did not offer protection against acetylsalicylic acid inhibition was supplemented by TRNOE studies which showed no significant change in the conformation of coenzyme bound to bakers' yeast G6PD inactivated with acetylsalicylic acid compared with the active enzyme. 5. Acetylsalicylic acid did not inactivate rat liver G6PD to the same extent as the bakers' yeast enzyme. Inactivation of rat liver 14 G6PD with C-HIAB was relatively slow and resulted in the partial labelling of many cysteine residues. 6. Sequence studies on rat liver G6PD resulted in the isolation of a peptide which was homologous in sequence to the bakers' yeast G6PD tryptic peptide containing the reactive lysine residue, although no evidence for special reactivity of the corresponding lysine residue in the rat liver enzyme was found. 7. A high degree of sequence homology was noted between rat liver G6PD and human erythrocyte G6PD. It was also established that the human erythrocyte G6PD sequence published by Beutler (1983) was incorrect due to the misalignment of tryptic peptides.

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