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Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes reversibly the oxidation and phosphorylation of D-glyceraldehyde 3-phosphate (G-3P) to 1,3-diphosphoglycerate (DPGA). Glyceraldehyde-3-phosphate dehydrogenase occurs widely and abundantly throughout nature. It comprises about 20% of the total soluble protein in yeast and up to 10% of the soluble protein from muscle, and the relative ease of its preparation from a wide variety of different species has contributed to its popularity among enzymologists, protein chemists, and X-ray crystallographers. Study of the active enzyme–NAD complex has been facilitated by the fact that uniquely among NAD-linked enzymes crystalline muscle GAPDH contains firm bound NAD. Pure crystalline GAPDH has been isolated from a number of different sources. Methods of purification have relied heavily upon its solubility as the enzyme-NAD …