Crystalline 6‐Phosphogluconate Dehydrogenase from Sheep Liver

6‐Phosphogluconate dehydrogenase has been obtained in the pure, crystalline form from sheep liver by an extensive modification of an earlier purification procedure. The absorption coefficient, A1%280, estimated from dry weight measurements, is 11.4. The amino acid composition is reported. The molecular weight of the enzyme from measurement by several methods is 94000 ± 2000, and there are two subunits in the molecule. From studies of the reaction of the native enzyme with 5,5′‐dithiobis(2‐nitrobenzoic acid) and p‐hydroxymercuribenzoate, there appear to be two reactive thiol groups per subunit which are essential for activity and are protected by 6‐phosphogluconate or NADPH.