J.E. Murphy

It has been found that a reagent containing sulphuric acid, ammonium molybdate and ascorbic acid may be used as a single-solution reagent for the determination of phosphate in sea water. Development of the molybdenum-blue colour is complete in 24 h at room temperature and in 30 min at 60° C; the colour is stable for at least 3 days. Beer's law is obeyed closely up to at least 500 μg PO 4 3− -P/1. The salt error is approximately 4% with sea water of chlorinity 19·3%. The interference due to either arsenate or silicate at their concentrations in sea water is negligible.

The replacement of aspartic acid by histidine at position 153 in Escherichia coli alkaline phosphatase results in a mutant enzyme that is remarkably similar to certain mammalian alkaline phosphatases that are activated by magnesium in a time-dependent fashion.These mammalian alkaline phosphatases have histidine at the position corresponding to 153 of the E. coli sequence.Here we report the three-dimensional structure of the mutant E. coli alkaline phosphatase with histidine at position 153.The structure reveals that the octahedral magnesium binding site has been converted to a tetrahedral zinc binding site with an imidazole ring nitrogen of His-153 as one of the ligands to the zinc.The alteration in metal binding caused by the mutation could explain the origin of the magnesium activation observed with the mammalian alkaline phosphatases.The structure also reveals differences in the mode of phosphate binding, explaining the enhanced phosphate affinity and the reduced activity of the mutant enzyme in the presence of zinc.Alkaline phosphatase (EC 3.1.3.1) is a nonspecific phosphomonoesterase that functions through a phosphoseryl intermediate (1) to produce inorganic phosphate and an alcohol.The alkaline phosphatase from Escherichia coli is a dimeric metalloenzyme that contains two atoms of zinc and one atom of magnesium per monomer.The x-ray structure