Marsh (1) has proposed a procedure for the estimation of inorganic orthophosphate (Pi) in the presence of ATP, which is purportedly designed to overcome errors resulting from molybdate-catalyzed hydrolysis of ATP (2). In a critique of methods currently used for the assay of Pi in the presence of labile phosphate esters, Marsh (1) states that the Lowry and Lopez method (3) is "peculiarly ill-suited for use with ATP," and gives as his source of information a publication by Weil-Malherbe and Green (2). However, this latter paper, although pointing out that the pH of 4 utilized in the Lowry and Lopez procedure (3) is also optimal for the molybdate-catalyzed hydrolysis of the terminal phosphate group of ATP, further indicates that such hydrolysis may not be extensive at the temperature (room) and molybdate concentration (0.17%) employed by Lowry and Lopez (3). In view of Marsh's statement (l), viz., " . . , the continued use of the molybdenum-blue type of reaction in homogeneous solution cannot be justified," we have re-examined our adaptation of Potter's modification (4-6j of the Lowry and Lopez method (3) that has, for many years in this laboratory, proved to be convenient and accurate for the estimation of Pi in the presence of ATP or other labile phosphate esters.
EXPERIMENTAL, RESULTS, AND DISCUSSION
The representation data of Table 1 show t,he results obtained when a solution of Na,HATP (Sigma Chemical Co.), stored frozen at pH 7.0 for one month, was analyzed for inorganic orthophosphate (Pi) by three procedures, i.e., Fiske and SubbaRow (5, 7), Potter's adaptation (4, 6) of the Lowry and Lopez procedure (3), and Marsh (1).