(3,4) have demonstrated that the pyruvate,phosphate dikinase reaction occurs by a three-site nonclassical Tri (Uni Uni) Ping Pong mechanism. A pyrophosphoryl-and phosphoryl-histidyl residue of the enzyme has been proposed to serve as a carrier between the three sites (4).A number of enzymes that have reaction mechanisms involving phosphate moieties covalently bound to enzyme are now known (5). The phosphate has been found linked to proteins through different types of linkage, including a phosphoester with the hydroxyl group of serine (6), a phosphoramidate to N-1 or N-3 of histidine (7-10), and an acid anhydride to a y-carboxyl group of glutamic acid (11).The phosphoramidate and acid anhydride type of bond may be considered high in chemical energy (12), and thus either of these might meet the requirement of phosphoryl pyruvate, phosphate dikinase. However, at the outset we could not exclude the possibility of ester linkage to serine because the phosphate of this type of bond has been known to be transferable to ADP with concomitant formation of ATP (13). Thus, in an appropriate micro-environment even a seryl-phosphate bond may be considered a "high energy" type.In the present work using pyruvate,phosphate dikinase from Bacteroides symbiosus, the type of covalent bond of phosphoryl enzyme has been investigated and it has been shown to be a N-3-phosphohistidyl residue of the enzyme. (14). The product was purified by chromatography on a Dowex 1-X8 column (Cl-, 2 X 25 cm) (1). The radioactive fractions were pooled, lyophilized, and stored frozen. The concentration of P-enolpyruvate was determined enzymatically by a pyruvate kinase-lactate dehydrogenase system and specific radioactivity was calculated in cpm/,mol by dividing the cpm in material that migrated on paper chromatography with authentic P-enol[2-'4C]pyruvate by the Amols of P-enolpyruvate in the aliquot. The product was 92-94% pure.
MATERIALS AND METHODS
ChemicalsSynthesis of Phosphohistidine. Phosphohistidine was synthesized by reacting potassium phosphoramidate and histidine (15). Phosphoramidate was prepared from diphenylchlorophosphate (16). The phosphohistidine was purified by chromatography on a 2.4 X 30 cm column of Dowex 1-X8 using a 2-liter linear gradient of 0-1.0 M KHCO3, pH 8.25. The fractions were assayed for histidine (17) and phosphate (18), following cleavage of the phosphoramidate by acidification of an aliquot to pH 1.0. The fractions were pooled, cooled in ice, and brought to pH 3.0 with perchloric acid. After CO2 evolution ceased, the pH was rapidly adjusted to 9.0 with potassium hydroxide, and the sample was filtered to remove the precipitated potassium perchlorate and lyophilized. Upon rechromatography the product (with some potassium perchlorate) gave a single peak, which was 3-phosphohistidine, as confirmed by chromatography of the product with a tracer amount of a mixture of 1-and 3-phospho[carboxyl-'4C]histidine that had been prepared according to Hays et al. (10).Purification and Assay of Pyruvate,Phosphate Dikinase. The enzyme ...